I ANIMALS AND PLANTS 37 



distinguished as " holophytic," and that from lower substances, 

 which, however, contain organically combined carbon, as " sapro- 

 ph}tic," for such are formed by the death and decomposition of 

 living beings. The third mode of nutrition (found in some 

 bacteria) from wholly inorganic substances, including free nitrogen, 

 has received no technical name. All three modes are included 

 in the term " autotrophic " (self-nourishing). 



Vegetal feeders have a great tendency to accumulate reserves 

 in insoluble forms, such as ■ starch, paramylum, and oil-globules 

 on the one hand, and pyrenoids, proteid crystals, aleurone granules 

 on the other. 



When an animal-feeding cell encysts or surrounds itself 

 with a continuous membrane, this is always of nitrogenous 

 composition, usually containing the glucosamide '•' chitin." The 

 vegetal cell-wall, on the contrary, usually consists, at least 

 primarily, of the carbohydrate " cellulose " — the vegetal cell being 

 richly supplied with carbohydrate reserves, and drawing on them 

 to supply the material for its garment. This substance is what 

 we are all familiar with in cotton or tissue-paper. 



Again, not only is the vegetal cell very ready to surround 

 itself with a cell-wall, but its food-material, or rather, speaking 

 accurately, the inorganic materials from which that food is to be 

 manufactured, may diffuse through this wall with scarcely any 

 ditticulty. Such a cell can and does grow when encysted : it grows 

 even more readily in this state, since none of its energies are 

 absorbed by the necessities of locomotion, etc. Growth leads, of 

 course, to division : there is often an economy of wall-material by 

 the formation of a mere party-wall dividing the cavity of the old 

 cell-wall at its limit of growth into two new cavities of equal 

 size. Thus the division tends to form a colonial aggregate, 

 which continues to grow in a motionless, and, so far, a " resting " 

 state. We may call this "vegetative rest," to distinguish it 

 from " absolute rest," when all other life-processes (as well as 

 motion) are reduced to a minimum or absolutely suspended. 



The cells of a plant colony are usually connected by very 

 fine threads of protoplasm, passing through minute pores where 

 the new party-wall is left incomplete after cell-division.^ In 

 a few plants, such as most Fungi, the cell - partitions are 



^ Similar]}', threads unite the cells of the colonial plant -Flagellate Volvox, 

 passing through the thick gelatinous cell-wall (pp. 126-127, Fig. 44). 



