THE A CONTRIBUTION TO THE ANATOMY OF GINKGO BILOBA. P. J. P. SHAW, A.R.C.S. (WITH TEXT-PIGS ).

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1 THE NEW PHYTOIiOGIST. VOL. VII., Nos. 4 & 5. MAY 31ST, A CONTRIBUTION TO THE ANATOMY OF GINKGO BILOBA. BY P. J. P. SHAW, A.R.C.S. (WITH TEXT-PIGS ). "VTOTWITH STAN DING the numerous speculations upon the J^ morphological nature of the " collar " in the ovule of Ginkgo none appears to have been based upon an adequate anatomical investigation of the organ in question. Most have been derived from a study of malformations, the phylogenetic value of which many modern botanists are disposed to doubt. It is hoped that the results put forward in the present short note may furnish a new standpoint from which to view the problem. The material consisted of about a dozen specimens of the female " flower" preserved in spirit. All the specimens showed two ovules, one fertile and one small aborted sterile ovule. The sections were cut by hand, stained with safranin, and mounted in glycerine jelly. Subsequently a model of the xylem was built up from the sections, after the method employed by Parmer and Hill in the case of Angiopteris. VASCULAR SYSTBM OP PEMALE PLOWER. As is well known, the female " flower " consists of a stalk or peduncle bearing two lateral stalked ovules. A series of transverse sections was cut from the base of the peduncle up into the fertile ovule. A transverse section taken from as near the base of the peduncle as the specimens allowed shows a ring of four collateral bundles, the bundles being definitely associated in pairs. A little higher up in the peduncle, the two bundles of each pair unite and form two collateral bundles facing one another (Text-fig. 16, i). Of these two bundles one is concerned with the vascular supply of the fertile ovule and the other with that of the frequently sterile

2 86 Fig. 16. For description seep, 87,

3 The Anatomy of Ginkgo biloba. 87 Text-flg. 16. Phloem is cross-hatched, tracheids cut transversely represented by series of dots, cut longitudinally by lines. Position of sterile ovule shown thus X A=:bundle of fertile ovule ; B=bundle of sterite ovule : V=anomalous tissue; C=cainbiuni ; H^" horns"; T.L.=tracheids cut longitudinally; T.S.=tracheids cut transversely ; M=metaxylem ; K=crushed parenchyma between two phloems ; D=place where "horns" unite. PX=median plane of the pair of ovules. Transverse sections. I. Bundles in peduncle. 2 Single bundle in pedicel points of origin of anomalous tissue marked by two dots. 3, 4. Lower part of collar anomalous vascular tissue present. 5. Commencement of approximation of "horns" some tracheids cut longitudinally. 6, 7, 8. Union of " horns" inverted vascular bundles splay outwards in the top of the collar. 9. Single concentric bundle in base of ovule. 10, 11. Splitting of bundle giving rise to two concentric bundles. ovule. The latter is usually small and in its passage up the peduncle gradually dwindles away and becomes lost in the mass of parenchyma and "transfusion tracheids " forming the bulk of the ground-tissue. In some cases in which the sterile ovule has developed to a rather greater extent this bundle can be traced as running definitely into it. It may be remarked that, as we proceed up towards the ovule, we find a gradually increasing number of " transfusion tracheids" intermingled with the ordinary parenchyma. These " transfusion tracheids" are short and are reticulately thickened; they form a very characteristic feature of the sections. One of the bundles has now been disposed of and we pass to the consideration of the other, which will eventually give rise to the vascular system of the large fertile ovule. In a section taken at the base of the collar, just above the sterile ovule, this bundle exhibits normal collateral structure, the xylem facing towards the sterile ovule (Text-fig. 16, 2). On the outer edge of the phloem there is a fairly continuous layer of large, thick-walled, parenchymatous elements stained with safranin. At one or two spots the ordinary parenchyma of the ground-tissue next to these large cells has an abnormal appearance. The cells are ^smaller, possess den^e contents with well-marked nuclei, and appear to be rapidly dividing. The divisions at first take place irregularly in all directions, but a little higher up they begin to assume a definite tangential direction. The result is that at one or two places on the outer side of the main bundle, next the phloem, a dividing cambium is produced. This cambium gives rise fo tracheids externally and to phloem internally, and several little vascular bundles with inverted

4 88 F.J. F.Shaw. orientation are seen to be scattered along the outer edge of the main bundle. Text-flg. 16, 3, represents a section taken from the lower part of the collar. Here there are three anomalous bundles in addition to the main bundle. Moreover in one case the cambium of the main bundle hns become continuous round its flnnu with that of the nearest inverted bundle. A little higher up, about the middle of the collar, the parenchyma on the other flank of the main bundle also becomes merismatic, and here likewise the cambium becomes continuous,with that of the nearest inverted bundle. Text-fig. 16, 4 shows the vascular tissue slightly crescent-shaped, the xylem of the main bundle being on the concave side of the crescent and the xylem of the inverted tissue on the convex. The cambium of the main bundle is continuous with that of the inverted tissue round the "horns" of the crescent these " horns " face towards the sterile ovule. At the back of the main bundle the inverted tissue consists of two small bundles. The crescent-shaped condition of the vascular tissue becomes more marked further up towards the ovule. The two ' horns' curve round and become much more prominent. So rapidly does this take place that the tracheids of the main bundle and of the inverted xylem at the " horns " run nearly horizontally through the parenchyma and are hence cut longitudinally in the transverse sections. The cambium remainscontinuous round the "horns" during this process (Text-flg. 16, 5 and 6). In Text-flg. 16, 7, the cambium of the inverted tissue appears continuous right round the convex side of the crescent. The space between the two cambiums isfliledwith phloem, and it is difficult to draw a line of demarcation between the phloem of the inverted tissue and the normal phloem, but remnants of large parenchymatous cells can sometimes be discerned between the two. The tracheids of the inverted tissue are, on the whole, running outwards towards the periphery and are cut longitudinally right round the outer edge of the crescent. The parenchyma enclosed between the " horns" is intermingled with numerous tracheids and forms a sort of metaxylem. As regards the main bundle the tracheids of this metaxylem are centripetal. In the upper region of the collar, the inverted tissue continues to splay outwards and disappears from the transverse sections; it persists longest at the "horns "(Text-flg. 16,8). The tracheids of the main bundle at each " horn," those on the inner side of the crescent, continue to run almost horizontally, and higher up in the collar they

5 The Anatomy of Ginkgo biloba. 89 gradually approach one another. At the extreme top of the collar they meet and the cambium of the main bundle becomes continuous all the way round (Text-flg. 16, 9). The number of " transfusion tracheids" in the general parenchyma is very large at this level. A single concentric bundle is thus formed at the base of the ovule; its central part consists of the metaxylem, mentioned above, and it is enclosed by the genuine xylem of the bundle, The latter shows a marked tendency to aggregate at two points on opposite sides of the bundle (Text-flg. 16, 9). The tracheids in one half of the bundle all appear to be running towards the region where the " horns" have recently united, in the other half of the bundle they trend towards a point exactly opposite. This behaviour of the tracheids is a foreshadowing of the splitting of the concentric bundle which takes place a few sections higher up in the ovule. Seeing that the two " horns," which united to form the concentric bundle, faced towards the sterile ovule, it is plain that the separation of this bundle into two takes place at right angles to the plane of the ovules the two resulting bundles lying in that plane (Text-flg. 16, 10). They rapidly divaricate and become flrst horseshoe-shaped and then mesarch and concentric (Text-flg. 16, 11). Ultimately they die out after passing up the side of the megaspore cavity for some little distance. The essential feature to which attention is drawn consists in the development of anomalous vascular tissue, with inverted orientation, throughout the region of the collar. This tissue is strictly conflned to the collar and takes no part in the vascular supply of the ovule. Besides the anomalous tissue the peculiar manner in which the main bundle becomes concentric is worthy of notice. Stress has already been laid on the fact that throughout this process its cambium remains continuous with that of the anomalous tissue; i.e., while the " horns " approach one another, during the passdge of the bundle upwards, the cambium remains continuous round the " horns." Since these approach one another fairly rapidly, as is shewn by the fact that their tracheids run longitudinally in the transverse sections, it follows that a sort of oblique gutter of cambium is formed on either flank of the main bundle. The outer wall of the gutter is the cambium of the anomalous tissue, the inner wall that of the main bundle. A consideration of the description together with the photograph of the model will make this clear (Text-flg. 18). / v"

6 90 F. J. F. Shaw. A series of longitudinal sections cut at right angles to the plane of the ovules affords a complete confirmation of the interpretation advanced above. Such a series is of course tangential to the main bundle. The sections begin on the side farthest from the sterile ovule hence we gradually cut through the main bundle and into the "horns." In Text-fig. 17, it the main bundle is shewn, flanked by two others with inverted orientation. Further across the main bundle the plane of section traverses the region where it is passing into the concentric state. A section here shows the main bundle with two lateral anomalous bundles; the cambium, however, is continuous, the latter appearing as wings of the former (Text-fig. 17, 13); all the tracheids mti longitudinally. Here is seen the first indication of the gutter of cambium, filled, of course, with phloem, as' described above^ In the succeeding sections, approaching the sterile ovule, the distance from the top of the main bundle to the bottom of the gutters decreases, this shows that the gutters are running obliquely upwards through the collar (Text-fig. 17,14 and 15). illll m U. Text-flg. 17. Longitudinal sections (tangential). 12, 13, 14. Sections passing through main bundle towards sterile ovule cambium of main bundle continuous with that of anomalous tissue. 15. Tracheids of anomalous tissue and those of main bundle at the " horns " cut transversely. 16. Union of "horns" above, the main bundle is giving rise to one of the smaller concentric bundles in the ovule.

7 The Anatomy of Ginkgo biloba. 91 It will be remembered that in describing the transverse sections it was pointed out that the tracheids of the " horns" were cut longitudinally. In the longitudinal sections these are of course cut transversely (Text-flg. 17, 15). It is easy to realise how, during the transition of the main bundle to the concentric state, the anomalous tissue encircles the newly-formed half of the main bundle. The Text-fig. 18. Photograph of model of vascular tissue in collar, model shows xylem only and is viewed from above and behind. " Horns" of main bundle point away from, and anomalous tissue points towards observer; white lines show curve of tracheids. Area marked M. filled with metaxylem (which is not shown in the model), hence the union of "horns" can be seen at D; the lighter area C is the cambium. Note on one side the oblique gutter G caused by union of cambium of the anomalous tissue with that of the main bundle. tracheids of the main bundle belonging to the "horns" are also cut transversely, and they are situated at the base and inner side of the gutters. The union of these tracheids, when the main bundle becomes concentric, is shown in Text-fig. 17, 16. Here they are seen running from either side of the main bundle across it to the centre, and at right angles to its length. CONCLUSIONS. The occasional occurrence of an apical bud between the ovules and the general anatomy, force us to consider the peduncle as a shoot bearing two lateral stalked ovules. Each ovule is attached to the peduncle by a short pedicel, and at the point of union of pedicel and ovule we have the collar. It is however difficult to account for the inverted vascular tissue of the collar if we consider it as a reduced carpel, a view which has often been expressed. The pedicel seems to have a petiolar structure relative to the main 8hoot, for the single vascular bundle which traverses it has xylem

8 92 The Anatomy of Ginkgo biloba. directed towards what would be the upper surface. The relationship is however obscured owing to the excessive development of one ovule, which comes to lie almost apically. The pedicel passes very rapidly into the collar with its inverted bundles. Now in the seed Lagenostonia Lomaxi it is stated that the pedicel has the structure of a petiole, and, what from our point of view is more important, in the cupule the vascular bundles are often inverted (Oliver and Scott Phil. Trans., Vol. 197 B, 1904). It is only fair to state however that the general impression in the minds of the authors seems to have been that the bundles were, as a rule, normally orientated although there was a considerable amount of variation. At all events for the present we would tentatively suggest that, in the light of anatomical investigation, the collar of the ovule in Ginkgo is better regarded as a vestigial cupule than as anything in the nature of a carpel. Perhaps when our knowledge of the Pteridosperm seeds with cupules is more extended we may flnd some in which the cupular bundles were, as a rule, inversely orientated. In this connection it is interesting to note that* at the base of the seeds of certain Cycads (C. revoluta, C. Rumphii) we have a state of affairs closely paralleled in the collar of Ginkgo, the single vascular bundle supplying the ovule being accompanied on its outer convex side by a number of inversely orientated strands (Worsdell, Annalsof Bot. XII., 1898, PI. XVII., Pig. 4). It seems therefore as if both Ginkgo and the Cycads might have been derived from forms with cupuliferous seeds, a conclusion which coincides with other evidence. By some writers the collar has been considered as an outer integument, but if we are to look for an outer integument in the lower Gymnosperms we are much more likely to flnd it in the double nature of the real integument of Ginkgo and the Cycads than in the " collar" of the former, which appears to be essentially a "cupule," an organ sui generis. Of course it may be urged that the vascular bundles described in the present paper simply indicate that Ginkgo originally possessed a vascular structure similar to that of the MeduUosese. This view however would take no account of the peculiar situation of the anomalous tissue and, further, it offers no explanation of the "collar." It should also be remembered that the seed is a large one, and this fact, taken in conjunction with the large development of " transfusion tracheids," renders it possible that the presence of additional vascular tissue may have a physiological, rather than a morphological, signiflcance.

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