LIFE FORMS AND SYN ECOLOGICAL UNITS 297 



Adaptability of Life Forms. — The leading idea of Raunkiaer's 

 system of life forms — the adaptation of plants to the unfavorable 

 season — requires the use of epharmonic adaptations for purposes of 

 classification. The life forms must reflect, as truly as possible, the 

 prevailing conditions of life. If this assumption is correct, then con- 

 siderable changes in the conditions of life must entail also changes in 

 the grouping of life forms. 



As far back as Schloesing (1869), it was proved that tobacco plants 

 grown in moist and in dry air showed very marked chemical differences. 



Table 33. — Percentage Composition of Tobacco Grown in Moist and in 



Dry A.IR 



This seems to show that moisture favors the production of starch. 

 Dryness, on the other hand, increases the amount of woody supporting 

 tissue. This agrees with common observation. A decreased water 

 supply causes, according to MacDougal and Spohr (1918), a trans- 

 formation of the polysaccharides into anhydride, that is, supporting 

 tissue, and so a xeromorphic structure results. In succulents the 

 conversion of the polysaccharides into mucous pentosans leads to 

 succulence, which is also interpreted as an adaptation to dryness. 



The influence of habitat conditions upon periodicity (duration of 

 the development and function of the individual organs) as well as upon 

 external and internal structure of plants is well known. We must 

 here forego the opportunity to restate the anatomical morphological 

 adaptive phenomena of plants, which are dealt with very thoroughly 

 in all handbooks of plant geography (see also pp. 101, 125). 



From the facts cited above it follows that the characteristics used 

 in the delimitation of life forms are influenced by various external 

 factors. They are predominantly phenotypical characteristics. This 

 explains why one and the same species may belong to different classes 

 of life forms under varying climatic conditions. Therophytes assume 

 occasionally a hemicryptophytic form in cold climates {Poa annua, 

 Viola tricolor and others). Chamaephytes may become nanophanero- 

 phytes {Coronilla minima, Fumana) when grown under subtropical 

 conditions, while phanerophytes in the high mountains are often 

 reduced to the chamaephyte form, and moist, warm regions favor 

 the growth of lianas in various classes of life forms. 



