GEKEKAL PHYSIOLOGICAL CONSIDEBATIONS. 543 



The principal Classes of Plants are characterized by respectively 

 possessing a peculiar type or plan of combination of the organs, having 

 not only a morphological but a physiological speciality. The type, more 

 or less recognizable, is a mark of the existence of a common law of in- 

 herited organization throughout each class. Within the limits of the 

 Classes exist almost infinite varieties of form, referable to morphological 

 laws which have been investigated in the First Part of this work. A 

 complicated but graduated and interconnected body of laws was there 

 shown to regulate the variations of forms in plants generally. 



Lastly, in the description of the Natural Order of plants, it will have 

 been recognized that there are still more special laws of development, 

 causing the existence of resemblance in limited groups of species; and, 

 beyond this, every species or kind of plant has its form and mode of life 

 more or less definitely fixed and regulated by its special law of organiza- 

 tion derived from hereditary descent, and modified in accordance with 

 external circumstances and the requirements of the plant. 



These reflections enable us to explain simply the terms higher and 

 lower classes or species of plants. In the Protococcus, consisting of a 

 simple cell, the specific law, that which determines the characteristic 

 foini, follows immediately on the first of those above indicated. In a 

 Conferva, the second and third are both involved ; and the specific law at 

 once succeeds these. Proceeding step by step, we shall find species in 

 which there is a diversity of forms of the cell and of tissues (higher 

 Algse) ; next, an additional diversity of organs (leafy Cryptoganiia) ; and 

 then come into play the laws of the physiological and morphological types 

 of combination of organs, which are most complicated in the Flowering 

 Plants, in the development of which, however, from the original germ, or 

 embryonal vesicle, we may trace, in a graduated series, the commencement 

 of the operation of the successively less general laws of organization. 



Xot only do different plants display great diversities in structure 

 and composition, but each individual plant offers more or less 

 diverse characters at different periods of life. 



Plants commence their independent individual life in the form of 

 a cell or a group of cells separated from a parent organism. In 

 the lower plants such cells, once fully developed, as spores or as 

 yonidia, are capable, under suitable circumstances, of growing up 

 into complete plants. In the higher Classes these cells (embryonal 

 vesicles, or the primary cells of a leaf-bud) go through the earlier 

 stages of development connected with the parent organism, and 

 are detached (as seeds or as bubils, &c.) already provided with rudi- 

 mentary organs of vegetation. 



Duration of Vitality. In those cases where the detached bodies 

 are products of simple vegetative cell-division, they often proceed 

 at once to grow up into new plants (gonidia, zoospores), but more 

 frequently their vitality remains latent for a certain definite period 

 (bulbils, spores of Mosses, Ferns, &c.) ; and when the body is a 

 result of sexual reproduction, it almost always remains for a more 





