PHYSIOLOGY 257 



y. An embryo with many cotyledons, as in the Pines, 



z. Stem and leaves of a Dicotyledonous Plant, showing 

 the reticulated structure of the leaves, and the manner in 

 which they are articulated with the stem. On comparing 

 this stem and leaves with the corresponding parts of a Mono- 

 cotyledonous plant, Fig. 226, u, the difference between them 

 will be apparent. 



Such are the very obvious distinctions between the two 

 great classes of Phaenogamous, or flowering Plants ; and so 

 far is it from being necessary to examine the seeds in order 

 to determine to which class any given plant belongs, it is 

 only required to look at a leaf, or even a fragment of a leaf, 

 or a little piece of the stem, in order to know in an instant, 

 whether the seed has one or more cotyledons. So invariable 

 are nature's laws, that the parallel, or reticulated veins of a 

 leaf, show us the monocotyledon, and dicotyledon, respective- 

 ly, with the same certainty as though we dissected the seeds 

 themselves. 



Contents of the cells in the cellular structure. In some 

 plants, or in some parts of the same plant, the cells contain 

 air, while in other plants, or in different portions of the same, 

 they contain, or transmit various fluids. 



In aquatic plants, the cells of the parenchyma inclose 

 bundles of sap vessels, while those which answer to the pith 

 of other plants, contain air only ; and in shrubs and trees, 

 although the cells of the pith contain an aqueous fluid, in 

 the young state of the twig, yet they contain only air at a 

 later period, when the twig becomes firm wood. 



The cellular tissue is found in some part of nearly every 

 vegetable. Those of the lower orders, as the Mushrooms, 

 are composed entirely of these cells, while they enter more 

 sparingly into the highest orders of vegetable organic struc- 

 tures, as the trees, and all phaenogamous plants. 



This tissue, besides, in different plants, containing fluids 

 and air, is the repository of mealy, resinous, sugary, oily, 

 acid, and saline secretions. It is the medium, also, by which 

 the elaborated sap is diffused sideways through the plant, 

 and it is even believed by some naturalists, that the func- 

 tions of secretion, and nutrition, are performed in these 

 vessels.' 



The influence of vitality in plants is beautifully displayed 



by this structure. The cells of the living plant, swollen by 



the fluids they hold, retain their figure, during life, and by 



processes which we cannot explain, or detect, change theso 



22* 



