178 MEMOIES FROM THE DEPARTMENT OF BOTANY OF COLUMBIA COLLEGE. 



Fifth: Has environment any influence on the development 'of different tissues? It 

 undoubtedl}' has, and appears in two Avays. 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, Ave notice that some species such as 

 P. alphmm., P.- Pennsylvanicum and P. aviculare have well developed hypoderma and 

 starch-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 groAV in 

 localities exposed to more or less extreme and sudden changes in temperature and 

 humidity. The latter flourish best in swamps or sheltered places, Avhere the changes are 

 less frequent and violent, and Avhere the soil is more generously supplied Avith 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 groAving in more exposed places. 



