68 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



2 ". In the monocotyledonous embryo it frequently happens 

 that the plumule is rolled up in the cotyledon, the margins of 

 which grow together, so that the whole embryo forms one 

 uniform mass 293 ; but as soon as germination commences the 

 margins separate. 



581. The radicle elongates downwards, either directly from 

 the base of the embryo, or after previously rupturing the in- 

 tegument of the base. Plants with the first character are 

 called ExoRHiz^s 295 ; with the second, ENDORHiz^; 298 2 ". 



582. The endorhizal embryo is very common in monocoty- 

 ledons ; the exorhizal, in dicotyledons. 



583. When the' seed is called into action, germination takes 

 place. The juices of the plant, which before were insipid, 

 immediately afterwards abound with sugar ; Ex. Barley ; and 

 growth commences. 



584. This growth is in the first instance caused by the 

 absorption and decomposition of water, whose oxygen com- 

 bines with the superfluous carbon of the seed, and is expelled 

 in the form of carbonic acid gas. 



585. As this phenomenon does not take place in full-grown 

 plants, except in the dark (258), so neither can it occur in 

 seeds, except under the same condition. Hence an embryo, 

 exposed to constant light, would not germinate at all ; and 

 hence the care taken by nature to provide a covering to all 

 embryos in the form of the integuments of the seed or of a 

 pericarp. 



586. As soon as the necessary proportion of carbon is re- 

 moved from a seed by the expulsion of carbonic acid, the 

 young plant begins to absorb food, and to grow by the pro- 

 cesses of assimilation and respiration already described (254). 



ACROGENS, OR FLOWERLESS PLANTS. 



587. Many plants not being increased by seeds, the result 

 of the mutual action of sexual apparatus (531), are flowerless, 

 and destitute of organs of fructification. 



588. Such are propagated by what are called organs of re- 

 production, which have no other analogy with the organs of 

 fructification than that both perpetuate the species. 



