236 BOTANY 



case where other conditions affecting growth remain the same by 

 night as by day. A too low temperature during the night may, how- 

 ever, completely counteract the accelerating influence of darkness upon 

 the growth. 



Just as the rays of light of different wave-length and refrangi- 

 bility were found to be of different value in the process of assimilation, 

 so growth is by no means equal in differently-coloured light. It IS TO 



THE STRONGLY REFRACTIVE, SO-CALLED CHEMICAL, RAYS THAT THE 



Fio. 197. — Two leaves of Ro.nunculus Pursltii. L, An aerial leaf ; W, a submerged water-leaf. 



(After Goebel.) 



influence of light on growth is due : the red-yellow end of the 

 spectrum acts upon many plants in the same manner as darkness. 



Moisture exerts a twofold influence upon growth. It acts as a 

 stimulus, and also, by diminishing transpiration, increases turgidity. 



Plants in damp situations are usually larger than those grown in dry places, 

 and in fact may differ from them in their whole habit and mode of growth. Direct 

 contact with water seems frequently to exert a special influence upon the external 

 form of plants. Amphibious plants, that is, such as are capable of living both 

 upon land and in water, often assume in water an entirely different form from that 

 which they possess in air. This variation of form is particularly manifested in the 

 leaves, which, so long as they grow in water, are finely dissected, while in the air 

 their leaf-blades are much broader (Fig. 197). The leaf-stalks and internodes also 

 often exhibit a very different form in air and water, and undergo the same abnormal 

 elongation as in darkness. This is especially noticeable in submerged water-plants, 

 whoso organs must be brought to the surface of the water (young stems and leaf- 

 stalk of Trapa natans, stem of Sippuris, leaf-stalk of Nymphaea, Nupliar, Hydro- 

 charis). Such plants are enabled by this power of elongating their stems or leaf- 

 stalks to adapt themselves to the depth of the water, remaining short in shallow 



