324 
Lee. — Observations on the 
type is also indicated by the variations not uncommonly found in the 
seedlings of certain species. To take a single instance, in one seedling of 
Heliopsis laevis , the tetrarch stage present in the others was entirely 
omitted. Many smaller variations were noted in other cases, all of which 
unite in giving the observer the impression that even the extreme ‘ types ’ 
are only variations on one method of transition. Throughout the group 
this sense of similarity is apparent, and, as in other cases where evolution 
has been at work, it is impossible to draw hard and fast lines between the 
two extremes. The case of Heliopsis laevis again demonstrates the useless- 
ness of seedling structure as an indicator of affinity. 
It is very difficult to determine the cause or causes of such variations ; 
and it is equally difficult to determine which type— diarch or tetrarch— 
of seedling structure is primitive and which derived. Many authors have 
discussed the general question of the evolution of seedling structure. In 
her 6 Theory of the Double Leaf-trace’ Miss Thomas (12) examines the 
evidence and concludes that the tetrarch type is primitive, and has given 
rise to the diarch by reduction. To the present writer this conclusion 
seems to require extreme conservatism on the part of the seedling : for, 
unless we assume that the first seedling had a certain type of structure, and 
that in the ages that have passed since it existed, either the structure has 
remained unaltered or has changed once and once only, the term ‘primitive’ 
becomes meaningless and the discussion futile. 
But is such an assumption justified ? Are we at liberty to assume 
that while the mature parts of plants have varied enormously, the young 
parts have suffered little or no structural alteration ? Comparatively little 
work has been published on the physiology of seedlings, but sufficient has 
been done to indicate that seedlings can easily be altered artificially. Evans 
(4), working on Luzula , showed that by sowing the seeds at different depths 
in the soil a hypocotyl of corresponding length could be produced ; and 
when other methods are used and other species experimented with, it will 
probably be shown that within certain limits the size of the seedling organs 
can be regulated. Apart, however, from mere speculation, Evans’s work 
indicates that seedlings can be made to vary by artificially altering 
the environment. There is no doubt that great changes have occurred on 
the earth’s surface during even recent geological times, and if this be so, it is 
highly probable that seedlings also have altered in accordance with the 
changed conditions. The different sizes and shapes which seeds have 
assumed, and the various functions adopted by seedling organs (in con- 
nexion with storage, transference of food material, &c.), indicate that the 
seedling is the reverse of conservative, and that it is really very susceptible 
to changes of all kinds. There seems, therefore, a high degree of proba- 
bility that not only have seedlings not been more or less at a standstill 
during recent geological times, but that they have been continually evolving. 
