LIFE OF THE WHOLE PLANT. 773 



Oak, Bean, Tesi(fig. 160, c, c), in which case they are said to be 

 hypogeal (from two Greek words signifying under the earth); 

 or, as is more commonly the case, they burst through the coats, 

 and rise out of the ground in the form of green leaves (^fig, 161, 

 c, c), in which case they are epigeal (from two Greek words 

 signifying upon or above the earth). In the course of develop- 

 ment, the cotyledons commonly separate, and the plumule comes 

 out from between them {figs. 160, n, and 161). In those cases 

 where they remain within the integuments, they sometimes be- 

 come more or less united, so that the embryo resembles that of 

 a Monocotyledon; but a Dicotyledonous embryo maybe always 

 distinguished from a Monocotyledonous one, by its plumule 

 coming out from between the bases of the cotyledons, and not by 

 passing through a sheath, as in Monocotyledons {fig. 162). 



The radicle of a Monocotyledonous embryo (see p. 126) 

 is itself prolonged downwards by cell-multiplication just within 

 its apex {fig. 230, a), to form the root. An embryo which 

 germinates in this way, is termed cxorhizal. 



As a general rule, seeds do not germinate until they are 

 separated from their parents, but in some cases, and more 

 especially when invested by pulp, as in the Gourds, Melon, 

 Cucumber, Papaw, &c., they do so before they are detached. In 

 the above plants such a mode of germination is altogether 

 exceptional, but in the plants of the natural order Ehizopho- 

 racese, as the Mangrove {fig. 235), the seeds commonly germi- 

 nate in the pericarp before being separated from the tree, in 

 which case the radicle is protruded through the integuments of 

 the seed and pericarp, and becomes suspended in the air, where 

 it elongates. 



CHAPTER 2. 



GENERAL PHYSIOLOGY, OR LIFE OF THE WHOLE PLANT. 



Having now examined the special or individual functions of 

 the different organs of the plant, we proceed to give a general 

 sketch of the whole plant in a state of life or action. In doing 

 so, we shall first notice the substances required as food by plants ; 

 then proceed to consider the function of absorption by which food 

 is taken up, dissolved in water, by the roots ; then the process 

 oi circulation, or more properly the distribution of the fluid food 

 thus absorbed ; next in order will be described the functions 

 of respiration and assimilation, the objects of which are tQ 

 aerate and elaborate the crude food or sap, which, when it 

 reaches the leaves and other external organs, is not adapted for 

 3d 4 



