370 EMBRYOLOGY 



while the embryo assumes a more or less flattened form, and, 

 at its wider free end, the future cotyledons appear as two 

 lobes (VII, Co.) between which the plumule (p.) develops. The 

 attached end of the embryo invariably becomes the radicle 

 (VIII, r.), which thus faces towards the micropyle. Such em- 

 bryos are to be found in seeds from almost ripe fruits (cf. IX). 

 The general method of embryo-development just described 

 is that characteristic of a large number of Dicotyledons, although 

 there are numerous differences in detail which are beyond the scope 

 of this book. In Monocotyledons (Fig. 217, A-E), where the 

 suspensor is often a more bulky structure (cf. especially Fig. 

 217, G), the single cotyledon (Co.) occupies the terminal position, 

 whilst the plumule (p.) arises towards one side. It will be 

 noticed that the embryos of Gymnosperms and Angiosperms do 

 not develop a special absorptive organ (foot), such as charac- 

 terises those of Bryophyta and Pteridophyta, although in a few 

 cases haustorial outgrowths arise from the suspensor. 



The stimulating effect of fertilisation is not confined to the 

 embryo, but also affects the ovule as a whole and the enveloping 

 ovary, whilst in some cases even adjacent parts of the flower 

 undergo considerable enlargement and change (e.g. the perianth 

 in the Mulberry, the receptacle in the Strawberry). The seeds 

 are often many times the size of the ovules from which they 

 sprang (e.g. Pea, Bean, etc.). The enlarged ovary- wall becomes 

 the pericarp, and the integuments, or integument, form the 

 testa, whose final texture differs greatly, according to the nature 

 of the fruit (dehiscent or indehiscent) and the means of dispersal. 

 The character of the testa varies enormously, both as to its 

 surface, which is often sufficiently distinctive in its sculpturing 

 (e.g. Corn Cockle, Agrostemma) to serve for purposes of identi- 

 fication, and as to its durability and thickness. The latter 

 feature is probably responsible for the slow germination of some 

 seeds. It may be noted in this connection that the seeds of 

 the Hawthorn and others, which are normally swallowed and 

 pass through the alimentary tract of animals, germinate much 

 more readily after having been acted upon by the gastric juices. 

 The embryo may absorb all the food-materials in the endo- 

 sperm before entering upon its resting-stage (exalbuminous seeds, 

 as in the Shepherd's Purse, Pea, etc.), or part of the endosperm 



