44 THE TISSUES OF PLANTS 



cell-community, whose operations are perhaps as necessary 

 to the healthy discharge of the vital functions of the plant 

 as that of any other organ. 



If we consider the contents of the cells in animals, we 

 shall find that they vary in their character, and that this 

 variation not unfrequently gives an apparent color to the 

 tissues. We have seen that in plants the various and 

 beautiful hues of flowers is produced by fluid coloring mat- 

 ters which are visible through the colorless walls of the 

 cells ; but in animal tissues, the coloring matter which is 

 called pigment, occurs in the cells in the form of granules. 

 The most striking examples of pigment cells occur in the 

 iris of the eye, in the freckles of the skin, which are pro- 

 duced by aggregations of brown pigment cells, and in the 

 colored spots on the elytra of many coleopterous insects, as 

 for instance, the genus coocinella, which is specifically 

 named according to the number of dark spots on Ihe 

 scarlet elytra. All the endless varieties of color observable 

 in the hair of animals, in the plumage of birds, and on the 

 wings of lepidopterous insects, are produced by aggrega- 

 tions of cells which contain coloring matter. The colors 

 of shells result from the same cause. Thus one and the 

 same law has overspread the animal and vegetable creation 

 with endlessly diversified hues. 



The cells of the animal tissues, exercise the same select- 

 ing power as the cells of vegetables, on the fluid which per- 

 meates their walls. The blood is laterally transfused 

 through the walls of the capillaries, and its constituents 

 pass in a molecular form through the parieties of the cells 

 contained in the meshes of the capillary network; each 

 cell acts on the blood in its own peculiar manner, selecting 

 its own proper formative material. Thus the pigmentary 

 cells select the coloring matter of the blood, rejecting 



