455 
Seedling Structure of Gymnosperms. Ill 
3. The seed-leaves are frequently unequal in size, the larger one over- 
lapping the tip of the shorter ; there is a marked tendency to the formation 
of lobes, especially at the apex ; and a short basal cotyledonary tube is 
formed in some cases, e. g. Encephalartos Altensteinii. 
4. In the Cycadaceae the cotyledons are more or less intimately fused 
by their ventral surfaces, especially in the upper parts which are embedded 
in the prothallus. 
5. As regards structure, stomata generally are present ; the mesophyll 
is homogeneous ; secretory cells and canals are common ; and the vascular 
bundles are mesarch or exarch in varying degrees. 
6. The number of bundles in each cotyledon varies (see table below) ; 
if the seed-leaves of a seedling are unequal in size, the smaller one generally 
has fewer bundles than the larger. 
7. In all cases the bundles are more numerous in the central region 
than in the basal parts and the tips ; or, in other words, the strands passing 
into the seed-leaves from the axis branch or dichotomize sometimes very 
regularly as in Ginkgo ; the branches may subsequently fuse together 
in the apical regions. 
Transition-region. 
8. In all cases the transition-phenomena take place rapidly, so that the 
hypocotyl throughout the greater part of its length shows root-structure. 
These changes are most rapid in Macrozamict spiralis and slowest in 
Encephalartos Altensteinii. Dioon edule and Stangeria sp. occupy an 
intermediate position as regards the features in question. 
9. In all the plants examined the vascular rearrangements take place 
within the hypocotyl. 
10. In no Cycad has a rotation of the protoxylem of the seed-leaf- 
traces been observed ; the amount of centrifugal wood is so small that only 
a slight rearrangement is required in order to bring the protoxylem into 
the exarch position. In Ginkgo such a rotation does take place within the 
hypocotyledonary axis. 
11. In Ginkgo each cotyledonary bundle, in dicotyledonous examples, 
gives rise to two poles of the root-structure ; in the tricotyledonous specimen 
examined each seed-leaf-strand gave origin to one pole, so that the resulting 
root-structure was triarch. 
12. In the Cycads examined — Macrozamia spiralis , Stangeria sp. y 
Dioon edule y and Encephalartos Altensteinii — the cotyledonary bundles 
fuse with the plumular traces, and ultimately form a central cylinder 
which consists in Macrozamia spiralis either of a central rod of xylem 
surrounded by phloem, or of an open U-shaped strand which soon 
closes to form a triangular-shaped vascular cylinder, as viewed in trans- 
verse section, enclosing a central mass of parenchyma ; in Stangeria , 
