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Hill and de Fraine. — On the 
attention to some of the more salient facts. Their structure is simple ; the 
mesophyll is homogeneous with its parenchymatous elements densely 
crowded with starch grains ; also stomata occur, a feature which has been 
fully considered by Wigglesworth 1 and Sprecher. 2 
Resin-ducts are present, they appear first at the corners of the cotyle- 
dons, but at lower levels several may be present (Diagram i, Figs. 2 and 6) ; 
also secretory cells are abundant. 
The chief feature of interest in the structure of the vascular bundles is 
their mesarch structure, a fact which has been recorded by Worsdell 3 and 
other observers. On the outer side of the soft bast is a band of fibrous 
elements precisely similar to the same tissue alluded to in the case of many 
of the plants already considered 4 (Diagram i, Figs. 3 and 5). 
Transition. 
Each seed-leaf has four vascular strands at the apex arranged in two 
pairs (Diagram 1, Fig. 1). In one plant examined a cotyledon had but 
three strands at its distal end ; but, at a lower level, an extra bundle 
developed next to the odd one, so that equality was constituted. Tracing 
these strands downwards, the individuals of each pair approach each other, 
and a union is brought about by the fusion of the fibrous elements on the 
outer side of the soft bast (Diagram 1, Fig. 2). This condition is maintained 
for some distance: the bundles, as viewed in transverse section, appear 
semi-circular in outline with the two xylem masses facing each other in the 
manner indicated in Diagram 1, Fig. 3. At the same time the wood has 
become more compact, and new xylem elements may appear between the 
centripetal tracheides ; this before the actual fusion of the two groups 
of vascular tissue has taken place. At a lower level the union is con- 
summated so that each cotyledon now has two mesarch bundles with the 
vascular arrangement indicated in Diagram 1, Figs. 4 and 5. 
This condition persists for some distance downwards : then, as the 
strands are traced towards the cotyledonary node, they are seen gradually 
to approach one another and ultimately to fuse, first by the fibrous elements 
(Diagram 1, Fig. 6), and then by the soft bast. The metaxylems are con- 
nected together by the protoxylem elements which are tangentially ex- 
tended ; they — the metaxylems — may or may not fuse (Diagram 1, Figs. 7 
and 8) and in this state the bundles enter the hypocotyledonary axis. 
It may be remarked here that some earlier authors (Worsdell, 5 Seward 
and Gowan, 6 and Sprecher 7 ) are all agreed upon the presence of two 
bundles throughout the main length of each cotyledon and the occurrence 
of one trace at the base of the seed-leaf, but they have not remarked upon 
1 Wigglesworth: The Cotyledons of Ginkgo biloba and Cycas revolnta (Ann. Bot., xvii, 1903). 
2 loc. cit. 3 loc.' cit. 4 Parts I and II, Ann. Bot., 1908, 1909. 
5 loc. cit. 6 loc. cit. 7 loc. cit. 
