266 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



The flowering specimen of Bidens beckii shown in Fig. 118 bears three kinds of 

 leaves. The lower, submerged part of the stem bears the deeply cleft leaves 

 typical of many aquatics. The upper part of the stem, however, which 

 formed above the surface of the water, has simple, nearly entire leaves. In the 

 intermediate region of the stem are found leaves that are intermediate in char- 

 acter/ The ordinary arrow-head {Sagittaria sagittifolia) , which grows in stag- 

 nant or slowly-flowing water, has arrow-shaped leaves with long petioles. If 

 the plants are grown entirely under water, then only linear leaves are formed, 

 but if the water level is not very high (Fig. 119), only the completely sub- 



Fig. 117. — Ranunculus fluilans. i, water form; 2, land form. 



merged leaves remain narrow, while the rest assume the usual arrow-shaped 

 form. There are numerous transition stages between these two forms. 



These observations lead to the conclusion that the form of the plant is 

 greatly influenced by the amount of available water. This conclusion is sub- 

 stantiated by direct experiment. If one specimen of an herbaceous annual is 

 grown with rather dry soil and atmospheric conditions, and if another is grown 

 in very moist soil and in a nearly saturated atmosphere, plants of very different 

 structure are developed. The experiment with dry conditions may be conducted 



d For a discussion of the conditional determination of leaf-form in aquatic plants, see: 

 McCallum, W. B., On the nature of the stimulus causing the change of form and structure in 

 Proserpinaca palustris. Bot. gaz. 34: 93-108. 1902. MacDougal, D. T., The determina- 

 tive action of environic factors upon Neobeckia aquatica Greene [Nasturtium lacustre A. 

 Gray]. Flora 106: 264-280. 1914- — Ed. 



