16 STUDY OF COMMON PLANTS. 



were distributed to the interior of the seed. The hilum is 

 in almost all cases a conspicuous feature, readily seen by 

 the unaided eye, or with the help of a lens. The chalaza 

 and raphe, on the contrary, are frequently obscured by 

 the growth of the seed-coats. The micropyle is the open- 

 ing between the seed-coats, readily seen in early stages of 

 development, but often not easily recognized from the out- 

 side of the mature seed. Its position -is most readily 

 determined by opening the seed and finding the radicle, 

 which, as already said, points toward the micropyle. 



The form of the seed is also determined largely by the 

 direction of growth of the ovule. In the majority of 

 cases, of which the castor oil seed is a good 

 termmedby example, the developing ovule turns upon its 

 direction of longitudinal axis in such a way as to take an 

 ' inverted position, so that in the mature seed 



the hilum and micropyle are close together, the chalaza at 

 the opposite end, and the raphe running the whole length 

 of the seed. Such seeds are said to be anatropous. 

 Others, as, for example, the seeds of stramonium, are 

 simply much curved, bringing both chalaza and micropyle 

 near the hilum, one on either side of it. This is the 

 so-called campylotropous form. In comparatively few 

 species, of which buckwheat is an example, the axis of 

 the ovule remains straight throughout its development, 

 and the seed is said to be orthotropous. Modifications, 

 particularly of the first and second forms, are of frequent 

 occurrence. Cf. Gray, Structural Botany, pp. 278, 279. 



Physiologically, seeds present many points of interest. 

 The arrangements for dispersal, for protection, and for 

 Physiological *^^ support of the embryo in germination are 

 adaptations, among the most important. 



A species generally has a better chance of survival if 



