Vegetable Pliysiology. 373 



which is given to a plant or animal in classification depends 

 upon the degree to which the principle of specialization, or divi- 

 sion of labour, is carried out in it. As the organization becomes 

 more complicated, its parts more mutually dependent, we say it 

 is higher ; and while, in regard to physiological functions, the 

 more the different actions are confined to distinct organs, the 

 more exalted becomes the character of the life ; so the greater 

 diversity that presents itself in the outward form, and in the 

 modes of combination of the organs, the higher becomes the 

 morphological character. Now the vital functions of plants are 

 so few in kind, and so simple, that their distribution among dis- 

 tinct organs only leads to a comparatively small amount of vari- 

 ation. The chief distinctions between the subordinate classes 

 of plants depend upon differences of form and arrangement of 

 organs which physiologically correspond, and are only unlike in 

 minor particulars, to which it is difhcult to assign any physio- 

 logical value. Then again the structural conditions may vary 

 almost indefinitely in complexity in the same organ within the 

 limits of a single class in both the lower and the higher types. 

 In the Algse the thallus, or vegetative mass, presents gradations 

 from the simple cell, or string of simple membranous cells, to 

 the enormous frond of Macrocystis 500 feet in length, where the 

 cells of which different regions of the thallus are composed, 

 exhibit considerable differences both in their forms and func- 

 tions. Among flowering-plants we find, in the same monocoty- 

 ledonous class, the palms with their magnificent organization, and 

 the duckweed of our pools, in which the physiological functions 

 are performed by organs constructed on an analogous type, but 

 in the latter case almost rudimentary in their internal organi- 

 zation. 



From these considerations it becomes evident that there can 

 be no serial arrangement of vegetable forms in a single graduated 

 scale. We do indeed find a progressive complexity or perfec- 

 tion in the types or plans which cliaracterise the great classes ; yet 

 these do not run into one another, but rather stand side by side, 

 exhibiting corresponding gradations, or running out from a com- 

 mon centre into radii of different length. 



The vegetable kingdom falls very naturally into two great 

 sub-kingdoms or regions, characterised at once by the outward 

 form (morphologically), by an essential diversity in the internal 

 structure, and by the different degree of specialization of the 

 functions (physiologically). In the lower group we can find no 

 physiological distinctions in the structures devoted to the vege- 

 tative life, the general mass of cellular tissue carrying on in com- 

 mon the processes of absorption, digestion, respiration, and 

 development. In the higher group there exists, well-defined in 



VOT.. XVIII. 2 c 



