390 



PHYSIOLOGY OF GROWTH. 



obliquely to the vertical, and therefore seeks, in virtue of its 

 geotropic irritability, to turn downwards in a curve. Seedlings of 

 various other Papilionaceae behave in these respects like the seed- 

 lings of Vicia; and it is further to be observed that the roots exhibit 

 the nutation referred to, not only in a moist atmosphere, but also, 

 although not to the same extent, when grown in loose earth or 

 sawdust. If in the seedlings of the Papilionaceae we consider 

 the posterior side to be that on which the convexity of the stem 

 lies (see Fig. 129, If), the anterior, F, that towards which our 

 roots always turn, then the median plane of the seedling corre- 



H 



FIG. 129. Seedling of Vicia 

 Faba. 



FIG. 130. Seedling- of 

 Phaseolus multiflorus, upper 

 portion. 



sponds exactly with that in which the two cotyledons are in con- 

 tact with each other. The fact that the curvature of the root, 

 resulting from the nutation of the hypocotyl and base of the root, 

 always takes place in the median plane of the seedling, must, for 

 obvious reasons, be carefully noted in studying the behaviour of 

 roots placed in a horizontal position. Thus, e.g., Vicia seedlings 

 on a horizontal surface must be placed so that they lie with their 

 right or their left side i.e. the outer face of one of their coty- 

 ledons on this surface. 1 



Interesting examples of nutation are also to be observed in 

 the first segments of the stem in many dicotyledons ; and we 

 will study these with some exactness, selecting for observation 

 Phaseolus multiflorus. If we split a seed which has been placed 



