38 THE TISSUES OF PLANTS 



altogether from the organism, as in the medusae or jelly 

 fishes. At length we arrive at animals whose bodies ai'e 

 made up of nothing but cells iii contact with each other, 

 and which permanently retain that uniformity of appearance 

 presented by every kind of tissue, whether animal or vege- 

 table, in the first stages of its development. Such is the 

 case with the vast' tribe of the infusorial aiuimaJs, so called 

 because they abound in decaying animal and vegetable in- 

 fusions. These animals move about in the water by means 

 of little hair-like organs at their surface, which are them- 

 selves merely modified cells. These creatures have been very 

 appropriately named protozoa, as they hold a corresponding 

 rank in the animal creation to the protophytes in the vege- 

 table. Thus animals and plants are alike in their gradually 

 increasing simplicity of organization, as we approach the 

 cell, — ^their primitive form and common starting point. 



The primitive rounded form of the cells is retained when- 

 ever they are loosely aggregated, as in the pith of most 

 herbaceous plants and the pulpy part of fruits ; in the more 

 compact tissues of the epidermis and the parenchyma of the 

 leaves they are angular and polyhedral. In the greatest 

 number of cases each utricle is compressed into the form of 

 a dodecahedron, and therefore necessarily exhibits a little 

 hexagonal cavity when seen in section. Occasionally these 

 dodecahedral cells develope with the greatest geometrical 

 regularity; most frequently, however, they are extremely irre- 

 gular in outline, some of their walls growing at the expense 

 of the others, which thus become greatly reduced insize or 

 even suppressed altogether, so that the cells exhibit on the 

 cross section pentagonal, or even cubical, as well as hexa- 

 gonal cavities. This is well exemplified in the epidermis of 

 Tradescantia discolor, the polyhedral cells of which are ex- 

 tremely irregular in outline. It is not difficult to see that 



