November, 1913. 



KNOWLEDGE. 



427 



language of research in this subject. It is generally accepted 

 that the central cylinder of the root in Phanerogams is far 

 more closely comparable to the leaf-trace cylinder of the stem 

 than to any one of the traces within it, yet when the com- 

 parison becomes detailed difficulties arise. For instance, 

 where there is a pith in the root it certainly forms part of the 

 stele, which is a solid cylinder sharply defined by the specialised 

 endodermis around it ; but the leaf-traces in the young stem 

 surround a massive cylinder of parenchyma exactly resembling 

 the parenchyma of the cortex with which it is in apparent 

 connection through the gaps between the leaf-traces. Even 

 the secondary formations in the stem do not completely 

 divide one system from the other ; when a specialised 

 endodermis is present it is not so clearly defined as in the 

 root ; in many cases it is not present, and in a few instances 

 there is an endodermis around each leaf-trace. However, the 

 stele in the stem of Phanerogams is not necessarily a morpho- 

 logical fiction, because in many stems its precise limits cannot 

 be determined, for morphology is not merely descriptive. If 

 we suppose that the stem stele in remote ancestors of the 

 Phanerogams was as well defined as that of the root, and 

 clearly comparable to it, we may attach a real morphological 

 meaning to the term when applied to modern Phanerogams, 

 provided we can show cause to believe that what we call the 

 stele in their stems represents the ancestral stele. Its tissues 

 will then have a history distinct from those of the cortex, 

 though not clearly separated from them. The burden of 

 proof, however, lies with those who assert that an apparently 

 continuous and uniform tissue can be separated into two 

 parts of distinct origin. 



The evidence advanced is of two kinds — one founded on the 

 comparative anatomy of stems and the other on the history 

 of the tissues of the individual plant. Schoute has collected 

 evidence to show that in the stems of Angiosperms a 

 specialised layer is commonly distinguished from adjacent 

 tissues either by the peculiar thickening characteristic of the 

 endodermis in the root, or by the presence of starch 

 in its cells. He shows that such a sheath surrounds 

 the vascular cylinder in a very large proportion of 

 Dicotyledons and in a majority of the Monocotyledons, 

 while among Gymnosperms it occurs but rarely ; and since 

 the Angiosperms in which this bundle-sheath is obscure or 

 wanting are commonly closely related to species in which it' 

 is perfectly well-defined he concludes that its absence in such 

 cases must be attributed to reduction. Allowing that such a 

 layer is as general among Angiosperms as Schoute believes, 

 doubt may still exist as to its homology with the endodermis 

 of the root, which is defined, not only by its thickened walls, 

 but also by the fact that the cells form the inmost rank of the 

 series of radial files distinguishing the inner cortex, while in 

 the stem the inner cortex cells are very rarely arranged radially. 



As regards the second class of evidence, that drawn from 

 the history of the tissues in the individual plant, we have 

 already seen that the differentiation of plerome from periblem 

 is far less definite at the growing- point of the stem than at the 

 root apex, and doubts have even been thrown on the identity 

 of plerome and periblem with stele and cortex respectively. 

 But we must now follow the development of the tissues of the 

 embryo into those of the seedling. The normal seedling 

 of all Phanerogams consists at first of cotyledons, hypocotyl, 

 and root, the plumular bud being still rudimentary. The 

 primary root lies usually in a straight line with the primary 

 stem or hypocotyl. The hypocotyl is commonly the first part 

 of the embryo to lengthen, and then its xylem is lignified a 

 little earlier than that of the root or even of the cotyledon ; 

 but when, as in many Monocotyledons, the base of the coty- 

 ledon lengthens first, lignification begins in that region and 

 advances through the hypocotyl to the primary root. The 

 investigation of the anatomy of the seedling at this epoch 

 becomes extremely important when the vascular system of the 

 root is compared with that of the stem, for in the seedling 

 we have a' complete and simple vascular skeleton which at one 

 end belongs to the primary root of the plant and at the other 

 to its primary stem ; hence there must be an intermediate 

 region in which stem structure passes into root structure, and 

 the method of transition should at least suggest, if it does not 



precisely determine, the relation in which they stand to one 

 another. For this reason great value has been attached by 

 anatomists to the transitional region of the main axis. Van 

 Tieghem showed that there are several types of transition 

 between root and stem, in all of which the xylem and phloem 

 bundles of the root are continued into the cotyledons or 

 plumule ; on their way through the hypocotyl they may divide 

 or be displaced, and the xylem bundles " rotate " (that is, they 

 turn on their axes until the protoxylem is internal), but all the 

 elements present in the root are continued upwards in regular 

 succession and are simply re-arranged in the upper part of the 

 seedling. Hence Van Tieghem considered that the steles 

 of root and stem are completely homologous. Gravis and 

 others, however, consider that there is no morphological 

 continuity in the hypocotyl between the vascular systems of 

 root, stem, and leaf : their traces are merely in contact 

 sufficiently intimate for physiological purposes, but there 

 is no true homology between the central cylinder of the 

 stem and that of the root. The third view is that of 

 Chaveaud, who agrees with Gravis that the presence of 

 external xylem is the rule in the hypocotyl and in the base 

 of the cotyledon, but considers that this external xylem 

 belongs to the primitive structure of hypocotyl and cotyledon 

 as well as to that of the root. 



As already stated, the vascular system of seedlings is first 

 differentiated in the hypocotyl, base of cotyledon, and base of 

 primary root. According to Chaveaud, in all these regions 

 the primitive stele is root-like, the xylem alternating with the 

 phloem and its development being centripetal ; but this 

 primitive formation is permanent only in the root and 

 commonly in the lower part of the hypocotyl also — in the 

 upper part of the hypocotyl and in the base of the cotyle- 

 dons the first xylem elements are fugitive and disappear 

 so early that as a rule they are missed completely by the 

 anatomist, who is apt to prefer well-differentiated material and 

 therefore to choose seedlings which are past their first youth. 

 Chaveaud therefore considers that there is an early phase in 

 the development of the seedling in which the stele of the 

 hypocotyl — at that time the only representative of the stem — 

 is developing on exactly the same lines as the stele of the 

 primary root, and is, in fact, continuous with it. At that epoch 

 each cotyledonary trace is also developing on the same plan : 

 it belongs to the same phase of evolution. In many 

 Dicotyledons the insertion of the cotyledons is the simplest 

 imaginable — the original stele of the hypocotyl divides below 

 the cotyledonary node and one half goes to each cotyledon. 

 Where this formation is clearly developed there cannot be 

 said to be any transition between stem and root structure, 

 since stem stele and root stele are continuous and their 

 steles are developing in the same way, while even the 

 leaf-traces of the first two leaves are on similar lines, 

 and their insertion therefore does not modify the structure 

 of the stele. The structure we associate with the stem 

 of Phanerogams appears as follows. In the transitional 

 region of the hypocotyl the first xylem elements — perhaps 

 only two or three at each pole — alternate with the phloem 

 groups. The elements next differentiated lie within them, for 

 development is still centripetal, but in two diverging groups. 

 The xylem ray is then shaped like an inverted V. Each arm 

 of the V approaches the adjacent phloem group as it travels 

 inwards, until the last-formed elements lie on the same radius 

 as the centre of the phloem group, but well within it. The 

 next elements are differentiated on that radius, but are 

 directed towards the phloem — development has become 

 centrifugal. These successive xylem formations are termed 

 by Chaveaud the alternate, the intermediate, and the super- 

 posed. The alternate elements are fugitive in this transitional 

 region : they commonly disappear as the superposed elements 

 become conspicuous. The intermediate xylem persists ; but 

 higher up in the hypocotyl the intermediate elements also 

 disappear as the seedling becomes older. Hence in seedlings 

 of a certain age we have endarch bundles at the top of the 

 hypocotyl, forming a stele of the stem type, and an exarch 

 stele lower down, which passes unchanged into the root, the 

 connection between the two being maintained by the inter- 

 mediate xylem of the transitional region. 



