CH. XIII] ENDOSPERM. 179 



In fig. 82, B 4, a central core of tissue is beginning to 

 be marked out in the centre of the embryo, as indicated 

 by shading ; this core, which is seen increasing in fig. B 5, 

 is the beginning of the vascular cylinder, which in the 

 embryonic condition is known as the plerome. These 

 two points the early appearance of the primary epi- 

 dermis, and the early appearance of the vascular cylinder 

 are the most important features in the histology of the 

 embryo. 



The growth of the embryo is provided for by a supply 

 of reserve material, stored up in a specially developed 

 mass of cellular tissue which forms inside the embryo-sac. 

 This, which is known as endosperm, is the product of the 

 secondary nucleus (s.e. fig. 80) of the embryo-sac. In 

 some cases, e.g. in grasses, the endosperm is formed in such 

 quantities that it is not all used in the growth of the em- 

 bryo ; thus in the mature seed the embryo is accompanied 

 by a mass of endosperm, which is not utilised until, on the 

 germination of the seed, the embryo begins to grow. In 

 the seeds which have been studied in earlier chapters, 

 namely those of the bean, the gourd and the sunflower, 

 the mature seed contains no endosperm. There the endo- 

 sperm has a temporary importance, supplying food to the 

 embryo as it grows: by the time the cotyledons have 

 reached their full size, the endosperm has disappeared, 

 and the whole cavity of the seed is occupied by the 

 embryo. What occurs on germination, when the embryo 

 wakes from its resting stage, has already been described 

 in Chapter II. 



122 



