STRUCTURE AND FORM OF SEEDS 5 



it except at the apex, where a small aperture, known as the 

 micropyle, is left. In the process of fertilisation the pollen- 

 tube passes down the micropyle to the oosphere which, enclosed 

 in the embryo-sac, approaches very close to the outer wall of 

 the ovule at this point. After fertilisation the oosphere 

 develops into the embryo, the rest of the embryo-sac becom- 

 ing filled with a cellular tissue, the endosperm, containing a 

 store of food for the young plant. This is, however, some- 

 times re-absorbed before germination, so that the embryo occu- 

 pies the whole interior of the mature seed. The original sub- 

 stance of the ovule, the nucellus, is often completely displaced 

 by the embryo-sac, and the subsequent development of endo- 

 sperm in its interior. One or two layers, however, may per- 

 sist and share with the integuments in the formation of the 

 seed-coats. Occasionally, as occurs in the Nymphseacese, 

 Piperacese, and some of the Scitaminese, a considerable portion 

 of the nucellus remains in the mature seed, sharing with the 

 endosperm the function of storing reserve material, or it may 

 quite take the place of the endosperm. 



There is considerable variety in the position and arrange- 

 ment of parts of the ovule and subsequently of the seed. 

 They are attached to the placenta by a stalk or funicle, the 

 point of connection with the funicle being called the hilum. 

 Vascular tissue supplying nutriment runs up the funicle into 

 the integuments, and the point reached before the nutritive 

 vessels ramify is the clialaza. 



In some cases usually where there is only one ovule in the 

 ovary or ovary-cell, as e.g. in Buckwheat or Nettle, the ovule 

 stands erect> and the micropyle is opposite the hilum, which in 

 this case coincides with the chalaza. The ovule is then said 

 to be orthotropous l or straight. But this, though the simplest, 

 is the least common form of seed. More often, as in the Bean, 

 Chickweed, and many others, the ovule during its growth is 

 curved on itself, so that the apex and the micropyle are brought 

 down very near to the base. Such ovules are known as cam- 

 pylotropous or curved. 



1 Figures of the various positions of the ovule are given in various text- 

 books ; see, for instance, Prantl and Vines' Text-book on Botany, ed. 4, p. 173 

 fig. 130 ; or Asa Gray's Structural Botany, p. 278. 



