founded on the Structure of their Seedlings . 81 
compared with the formation of two cotyledons (Darwin, 1. c.). 
It is true that in time the extra assimilating surfaces will 
more than repay the cost of their production, but time may 
fail the geophyte which dares not risk being caught by the 
bad weather unprepared. 
These considerations have led me to look upon the Mono- 
cotyledon as an organism adapted primarily to a geophilous 
habit. The single cotyledon has been shown to be connected 
with this way of life in some Dicotyledons, and many of the 
features which distinguish Monocotyledons from Dicotyledons 
may be explained as having been formed under the conditions 
I have just described. Since I have adopted this view as 
a working hypothesis, the purpose of many details in the 
structure of Monocotyledons which had puzzled me before 
has become comprehensible. 
Distribution of the Bundles in the Stem . 
An erect subterranean axis with much shortened inter- 
nodes and crowded with the sheathing bases of leaves 
must inevitably receive a number of traces from each leaf, 
and these traces — entering the axis in segments of its cir- 
cumference corresponding to the breadth of the leaf-base — 
would naturally arrange themselves in more or less complete 
concentric circles. Mr. Henslow has well described the 
process by which this might occur (15, p. 512), but his 
suggestions are much more applicable to a short vertical 
subterranean axis such as that found in the four-year-old 
seedling of Podophyllum (Holm, 19) than to the rhizome of 
Nymphaea. In a squat underground root-stock secondary 
growth of the xylem in thickness would be useless : the 
bundles are essentially channels of communication between 
the leaves and the roots, and they are not required to support 
a great mechanical strain. Thus the extra-fascicular cambium 
would first disappear — as it has done in Podophyllum — and 
later the cambial zone from each bundle. The bundles of 
Podophyllum possess distinct fascicular cambium, but they are 
isolated from each other by the well-marked bundle-sheaths. 
