178 MEMOIRS FROM THE DEPARTMENT OF BOTANY OF COLUMBIA COLLEGE. 
Fifth: Has environment any influence on the development ‘of different tissues? It 
undoubtedly has, and appears in two ways. On page 12 I grouped the subgenera 
according to their habitats and later we saw that the anatomical structure gave the same 
grouping, which is to say that the same or like environment directs the development of 
tissues along similar lines. In the second place, we notice that some species such as 
P. alpinum, P. Pennsylvoanicum and P. aviculare have well developed hypoderma and 
stareh-ring, while others, as P. scandens and P. arifolium, have these tissues but weakly 
developed. The contrast is so striking that it demands an answer. The first grow in 
localities exposed to more or less extreme and sudden changes in temperature and 
humidity. he latter flourish best in swamps or sheltered places, where the changes are 
less frequent and violent, and where the soil is more generously supplied with moisture. 
With a plentiful supply of water in the soil, a fairly even temperature and moisture in 
the atmosphere, the plants need not be protected as thoroughly from transpiration as 
those growing in more exposed places. 
to oD 
