XII.] THE DIRECTION OF DEVELOPMENT. 137 



matter which is retained in the old organism, and ministers 

 to the growth or renewal of its tissues. 



The next distinction is that into cellular structure, which Separation 

 is generally in animals, and probably always in vegetables, and^ascu- 

 that in which the nutritive processes are actively going on ; lar struc- 

 and vascular structure, which, in its simplest form, consists 

 of rows of cells which have been joined to each other by Cells unite 

 the removal of part of their walls (as rooms may be thrown vessels! 

 together by the removal of partitions), so as to form tubes 

 for the conveyance of nutritive fluids from one part of the 

 organism to another.^ Vessels are not needed, and con- 

 sequently they do not exist, in those organisms where all 

 the parts are alike in structure, and equally well situated for 

 obtaining nourishment. For this reason they are not found 

 among the Algse, every part of which is able to derive its 

 nourishment separately from the water ; but some rudiment 

 of vascular structure is found in all air-breathing plants, 

 except the simpler lichens and fungi,^ in which any 

 movement of fluid that may be necessary takes place by 

 permeation. For all tissues in which vital processes are 

 going on are permeable by water. 



The reproductive organs, and the vascular or circulatory 

 organs, which are thus separated from the general nutritive 

 system, continue, notwithstanding, to form part of the 

 organs of nutritive or vegetable life. Biit in the animal 

 kingdom, and in a few instances in the vegetable kingdom 

 also, a still more profound distinction is established. We 

 have seen that all organisms transform matter and energy ; 

 that the differentia of life consists in the power of effecting 

 a peculiar set of transformations of matter and energy. 

 The peculiar characteristic of animals, though some vege- ^.^P^ra- 

 tables share it to a slight extent,^, is the possession of a animals, 



1 The canals by which the substance of Sponges is traversed can 

 hardly be called structure at all ; and what they circulate is not saji or 

 blood, but only the water from which the sponge derives its nourishment. 



2 Carpenter's Comparative Physiology, p. 671. 



3 Cohn has found the structure of the filaments of the stamens of the 

 thistle to be closely analogous to that of involuntary muscle. For about 

 twenty-four houi's the anthers, if touched, show a peculiar twisting 

 motion, and extrude pollen. (Medico-Chirurgical Review, January 1864.) 



