20 



THE ANIMAL ORGANISM 



nourishment from the earth (the fact that it also obtains food from 

 the air is less generally known), (3) that it is fixed in one place 

 and does not move about — usually, indeed, does not move at 

 aU. 



I. The green colour of plants is due to the presence of the 

 substance known as chlorophyll. This is contained in protoplasmic 

 structures known as chloroplasts, which 

 in the green cells of the higher plants 

 are usually numerous and lens-shaped 

 (Fig. 7). Chlorophyll is a mixture of 

 four or more complex compounds of 

 carbon, hydrogen, oxygen, and nitrogen, 

 some of which contain in the molecule 

 an atom of magnesium. It is only found 

 in those parts of plants which are 

 exposed to sunlight, and is never found 



,cur. 



-^ j\ ^t) 



Fig. 7. — Plant cells. 



A, A small portion of green tissue from a plant. B, Part of a section through a leaf. — From Godwin. 

 a.s., Air spaces between the cells ; ch., chloroplasts ; c.k'., cell wall ; cu., cuticle ; ep., epidermis surface 



layer of cells ; nw., nucleus ; ppm., protoplasm ; s/., stoma (opening through which air enters) ; vac. 



vacuole (space containing fluid). The arrows show the paths of diffusion of carbon dioxide. 



in multi-cellular animals, except in certain cases where minute 

 green plants live embedded in the transparent protoplasm of 

 animal bodies, as in the green hydra (p. 89). At the same time it 

 must be remembered that certain plants, such as the Fungi, have 

 no chlorophyll, while many of the simplest animals, the Protozoa, 

 do have it (Chap. 3). 



2. More important than the mere presence of chlorophyll is 

 its function in the body, which is connected with the nutrition 



