22 



THE ANIMAL ORGANISM 



3. I'Yom the mode of nutrition of plants there follows the 

 tliird character which we have marked in them. In the great 

 majority of animals food must be either sought by locomotion 

 or at least seized by other active movements, as it is, for instance, 

 in a sea-anemone or Hydra (Chap. g). In plants, on the other 

 hand, not only is this necessity absent, but, since it is desirable 

 that they should expose as great a surface as possible to air 



.-Cv. 



A 



B 



Fig. 8. — Chlamydomonas, a minute, motile plant. 



.A. Ordinary individual. B. B', Two stages in the conjugation of gamrtesof equal sire (isogamy) ; C, C, 



Two stages in the conjugation of gametes of diflereut sizes (anisogauiy). The conjugation is ' head on' 



in each case. 

 cv.. Contractile vacuoles ; ck., chloroplast ; cm,, cuticle of cellulose, e, ty^ spot -.ft., flagelluui ; t%u., nucleus: 



HI*'., nuclei ol two gametes about to fuse ; f>yr., p\Tenoid (a protoplasmic lK)dy which is ooncexned in 



starch formation) ; s.g., starch grains.. 



and water for absorption — as they do, for example, in leaves and 

 roots — the shape of their bodies is necessarily such as to be an 

 actual hindrance to motion. Thus in most plants active motion 

 is restricted or absent, and muscular and nervous tissues are not 

 found in plant bodies. Certain microscopic aquatic organisms, 

 however, chielly unicellular Algie, are exceptions to the rule 

 that locomotion accompanies the animal mode of nutrition only. 

 Though they have one or more chloroplasts and nourish them- 



