612 BRITTON: VEGETATION OF THE 
ing lamellae. The phloem surrounds this tissue. Endodermis is 
still present, though secondary thickening has evidently begun, in 
the roots examined. Cells of the cortex are hexagonal and pen- 
tagonal in shape, are very large and have thin walls. The struc- 
ture of the root is quite similar to the root-structure of the other 
members of the Compositae that have been examined in preparing 
this paper. 
Diameter of root, .51 mm. 
Oy ‘* central cylinder, 1 .28 mm. 
y 
= ‘* largest vessel, 33 
THE STRUCTURE CONSIDERED IN RELATION TO THE 
ENVIRONMENT 
It is commonly known that the different kinds of plants differ 
in their minute structure as well as in their external gross appear- 
ance, though in most cases to a less marked degree. Nearly all 
of the works on structural botany contain figures illustrating the 
various plant tissues and forms of cells. The functions of most 
of these specialized tissues have been known for many years, but 
the variations were regarded as properly belonging to different 
species of plants instead of as special adaptations for the benefit of 
the plant that might finally lead to the erection of species. It has 
been. during ‘the last few years only that we have looked upon 
plants as being capable of great modification by environment, even 
as regards external characteristics. Much more recently has it 
been shown that histological differences, which fundamentally 
precede gross external differences, are the result of environment. 
Under favorable conditions perhaps we do not notice the changes 
so much, but when the conditions become unfavorable it will 
always be found that certain plants have been able to live where 
others have perished, and all because of slight variation in some 
direction which proves beneficial. So natural selection acting 
through many generations has preserved these beneficial varia- 
tions, which through the laws of heredity and by the aid of con-— 
tinued natural selection have reached a higher and higher devel- 
opment. 
Thus we have in aquatic plants specially developed air-passages 
and bladder-like floating tissues. Hydrophytes of fresh water also 
differ from those of salt water (halophytes) in their cell structure. 
