TROPISMS. REPRODUCTION 87 



chloroform. But in most cases we do not as yet know 

 nearly enough of the chemistry and physics of the living 

 cell to furnish a detailed explanation of the causes and 

 course of the particular response in every case. 



When the whole organism moves in response to an 

 external stimulus, the motion is called a taxis* When, 

 as in a fixed plant, an organ bends in response to a 

 stimulus, the motion is called a tropism. 2 For instance, 

 motile unicellular green plants (e.g. Chlamydomonas, 

 Chapter XII) are positively phototactic 3 in light of 

 weak or medium intensity coming from one side 

 i.e. they move towards the source of such light. Motile 

 unicellular organisms in general are chemotactic, either 

 positively or negatively, i.e. they move towards or away 

 from the source of various chemical substances diffusing 

 from a given source, according to the nature of the 

 substance. The tips of the branches of the stem and 

 root of a higher plant are phototropic, geotropic 4 and 

 hydrotropicfin different cases, bending towards or away 

 from a source of light, the direction in which gravity 

 is acting, or a source of moisture. Thus the tips of 

 roots are positively geotropic and hydrotropic, but 

 negatively phototropic, and so on. Of the detailed 

 effects of other stimuli, e.g. electrical stimuli, on the 

 protoplasm of cells we do not know enough to speak 

 confidently, though we know that such effects exist. 



(10) Reproduction may be defined as the production 

 of a new individual or new individuals from pre-existing 

 ones. It has been usefully defined as discontinuous 

 growth, since the formation of a new individual always 

 involves processes of growth, and this growth is 

 discontinuous, for it begins in the parent organism 



1 Greek rd^iQ, a disposition. Greek rpoVog, a turn. 



3 Greek 0djg, ^coro'g, Jight. 4 yfj, the earth. 5 vdo)p, water. 



