GENERAL PHYSIOLOGICAL CONSIDERATIONS. 547 



becoming in turn the parent cells of a new generation. The permanent 

 cells become parts of parenchymatous, prosenchymatous, or vascular tis- 

 sues in the vegetative organs, or parts of reproductive structures in flowers 

 and fruits. Thus they run through a course of life dependent in each 

 case on the laws of development of the plant, according to which its 

 organs have a shorter or longer duration. The death of the organ or tissue 

 in which they exist results from the cessation of the vital activity of the 

 cells according to these laws ; and their decay, from the now unopposed 

 operation of simple chemical forces. 



Vital Phenomena. The principal vital phenomena exhibited by 

 plants are connected either with the maintenance and increase of 

 the individual organism, or with the production of special structures 

 endowed with the power of growing up into new individuals when 

 thrown off by the parent. 



We say the principal vital phenomena, because there are some which 

 we cannot strictly affirm to belong to either of the above classes, although 

 there can be but little doubt that they are in some way related ; of these 

 are the movements of plants like the Sensitive-plants, the folding up of 

 leaves or flowers, &c. 



Vegetation, Nutrition. The processes of vegetation, or growth 

 of the individual, are, from the peculiar organization of vegetables, 

 connected with the processes of reproduction, properly so called, 

 by the phenomenon of multiplication or propagation through 

 natural or artificial separation of portions of the structure which 

 might remain and form branches of the parent stock. This vege- 

 tative propagation, distinct in important anatomical and physio- 

 logical characters from sexual reproduction, is found in all classes of 

 plants, and from its importance in relation to cultivation deserves 

 separate consideration. 



The construction of plants from a number of like parts more or less 

 physiologically independent allows of their being increased by mechanical 

 subdivision of the parent " stock," which is effected by making cuttings, 

 &c. The same occurs in the propagation of plants by bulbs, tubers, &c. 



The Vegetative processes of plants are divisible into several 

 heads, which, however, present many points of interconnexion. 



Nutrition, properly so called, can only be said to go on in the 

 protoplasmic matters found in the interior of cells, since it is these 

 substances alone that exhibit phenomena of consumption and repara- 

 tion. The tissues of plants are, under ordinary circumstances, 

 never renewed ; the only changes which they undergo are stages 

 of progressive development or growth, succeeded sooner or later 

 by decomposition. 



Development or organization constitutes the most striking mani- 

 festation of the vegetative action ; but this is a final result, pre- 

 pared from, and incessantly accompanied by, phenomena which are 



2N2 



