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Lee. — Observations on the 
however, there is much variation ; near the top of the hypocotyl the vascular 
bundles enlarge, while a little lower down they decrease to a very small 
number of elements in each case before finally increasing at a still lower 
level and attaining a comparatively large size. This is only one out of 
several instances where fluctuations occur in the size of the bundle in the 
hypocotyl. Everywhere in the latter organ, apparently, the vascular 
bundles do the same work, yet fluctuate in size. 
Finally, from a careful study of a large number of seedlings, and after 
making measurements into which it seems impossible to read anything like 
constancy or uniformity, the present writer is forced to the conclusion that 
other factors are at work in addition to those which are purely physio- 
logical. It is possible that two sets of factors are constantly influencing the 
seedling : (i) those factors which are purely physiological, and which are 
concerned with changes in the environment ; and (2) factors which are 
connected with the nature of the organism. 
It seems impossible to avoid the impression that changes occur in the 
seedling which cannot be correlated with changes in the environment. 
These changes in the seedling, due to the nature of the organism, occur 
continuously, and are probably to a certain extent guided by the first set, 
or physiological factors. But even when the latter are quiescent — as they 
must be when there are no changes in the environment — the innate factors 
are probably always active, and are constantly producing morphological 
changes in the various seedling-organs. The two sets of factors will some- 
times be antagonistic, in which case the physiological factors, which are based 
on the immediate needs of the plant, will be dominant and will determine 
the changes which occur. When the physiological factors are quiescent, 
and new structures are produced by the innate factors, so long as these new 
structures do not unfit the seedling, they will persist ; but when a new 
structure begins to clash with the environment the physiological factors will 
be brought into play. 
In general, seedlings are limited to two extremes of vascular structure, 
the diarch and the tetrarch. The course of evolution is probably not to be 
represented by a straight, nor by a zigzag line, but by a spiral with a large 
number of turns, the spiral representing the fluctuations between the diarch 
and tetrarch structure, the axis the general line of advance of the seedling. 
Summary. 
1. The present paper deals with the seedling structure of about fifty 
species belonging to the Compositae. 
2. All the seedlings belong either to the diarch or to the tetrarch type. 
3. Variations in vascular structure occur not only in nearly related 
species, but in different examples of the same species, indicating that 
seedling anatomy is of no value in questions of affinity. 
