6 VEGETABLE PHYSIOLOGY. [LESSON 2. 



28. SECRETION, in plants, consists in the process of separating 

 from the sap materials differing from itself. This function is dele- 

 gated to cells, in the Vegetable Kingdom, but the allied function in 

 the Animal Kingdom is performed by glands. In them resides the 

 power of secerning or secreting from the blood, substances entirely 

 different from it, or any of its constituents, as bile, saliva, mu- 

 cus, &c. 



29. It is very important to remark in regard to all the cells thus 

 actively concerned in the Vegetative functions in plants, by which 

 the development and extension of the permanent fabric is provided 

 for, that they have but a very transitory life as individuals. 



30. The Absorbent cells at the extremities of the rootlets are 

 continually being renewed ; some of the old ones dying and decaying 

 away, whilst others are converted into the solid texture of the root, 

 and thus contribute to its progressive elongation. 



31. In the process of ABSORPTION, another broad distinction 

 occurs between Plants and Animals. In the former kingdom, this, 

 like all other vegetative functions, is performed by cells ; but in the 

 Animal Kingdom the imbibition, or absorption, is performed by 

 special vessels, as the lacteals (laCta, milk,) and lymphatics. 



32. Of the transitory duration of the Assimilating cells, we have 

 an obvious proof in the fall of the leaf; which takes place at inter- 

 vals, to be followed by the production of a new set of cells, having 

 similar functions. 



33. The fall of the leaf results merely from the death and decay 

 of its tissue ; as is evident from the fact, that, for some time pre- 

 viously, its regular functions cease, and that, instead of appropriating 

 carbon from the atmosphere, there is a liberation of carbonic acid (a 

 result of their decomposition) in large amount. Thus, the process 

 takes place in evergreens equally with deciduous (falling off) plants ; 

 the only difference being, that the leaves in the latter are all cast off 

 and renewed together, whilst in the former they are continually being 

 shed and replaced, a few at a time. 



34. The Secreting cells have usually a like transitory duration ; 

 being destined to give up their contents by the rupture or liquefac- 

 tion of their walls, whenever called upon to do so, by the demand 

 set up in the growing parts of their neighborhood, for the peculiar 

 products they have set apart. 



35. Not only are the proper organic functions of all Plants thus 

 dependent upon the agency of cells, but their Reproduction is so 

 likewise. 



