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Ethel N. Thomas. 
features of a few Cycad seedlings, convinced me of the importance 
of further examination of these forms. The account given of 
Dioon edule recalled very vividly many Dicotyledonous seedlings, 
while the description and diagrammatic figures of Cycas siamensis 
corresponded exactly to the features already found in Liriodendron 
tulipifera. 
Dioon has four bundles in each cotyledon, and these form a 
tetrarch root, exactly as in the Dicotyledons of this type. Cycas 
siamensis has also four bundles, but as in Liriodendron tulipifera 
TETRARCH TYPE. 
Fig. 4. Transition features in a typical Cycad. 
the lateral bundles, although they enter the hypocotyl, fuse with 
those from the opposite cotyledon, and even begin to assume root 
characters, nevertheless die out, so that the root is diarch. It is 
impossible to avoid the conclusion that in both cases we have a 
stage in reduction from the tetrarch to the diarch type, particularly 
when it is taken in conjunction with other evidence to be considered 
below. 
A fair number of Cycads have now been examined, and all 
the available information gathered together. From this it appears 
that Cycad cotyledons are almost without exception destitute of a 
midrib, and have four or six bundles at their base; that the 
transition features take place so near the Cotyledonary node 
that the hypocotyl is root-like in structure; and that the primary 
root is in most cases tetrarch, with a strong tendency to reduce 
near its apex to diarch. The genus Cycas would appear to be most 
abarrent, for while C. revoluta forms a tetrarch root, C. siamensis 
as described above forms a link between C. revoluta and C. 
Rumphii, which is diarch through the absence of inter-cotyle 
donary poles. Further C. revoluta may have its two central 
bundles replaced by a single one, which however plays the part of 
two. 
