790 PHYSIOLOGY. 



a Monocotyledon ; but a Dicotyledonous embryo may be always 

 distinguished from a Monocotyledonous one, by its plumule 

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

 passing through a sheath {fig. 15). 



The radicle of a Dicotyledonous embryo (see p. 120) is itself 

 prolonged downwards by cell-multiplication just within its apex 

 {fig. 222, a), to form the root. An embryo which germinates in 

 this way is termed exurhizal. 



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. 229), 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. 



GENEEAL 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 recjuired as food by plants ; 

 then proceed to consider the function of absorption by which food 

 is taken up ; then the process of circulation, or more properly 

 the distribution of the fluid food thus absorbed ; next in order 

 will be described the functions of Tes])iration and assimilation, 

 the objects of which are to aerate and elaborate the crude food 

 or sap, and adapt it to the requirements of the plant : and, lastly, 

 our attention will be briefly directed to the functions of dcvclop- 

 ment and secretion. 



Section 1. — Food of Plants and its Sources. 



The various substances required as food can only be ascertained 

 by determining the elementary composition of the parts and 

 products of plants ; for as plants have no power of forming these 

 elements for themselves, they must have derived them from 

 external sources. 



As plants are commonly destitute of locomotion, being fixed to 

 the soil or to the substance upon which they grow, or floating in 

 water, they must obtain their food from the media by which 



