4 CELL GENETICS 



properties. This may be spoken of as differentiation-in-space-and- 

 time. With higher organisation (both in the Protista and 

 elsewhere) external conditions have less and less to do with the 

 direction of differentiation of particular cells although they continue 

 to modify its rate and consequently affect the result in detail. We 

 may say that, in general, the greater the internal capacity for 

 differentiation the less the special reaction to external changes. 



2. REPRODUCTION 



(i) Asexual Reproduction. Growth and reproduction equally 

 depend on increase in size and in organisms having a differentiation 

 of their substance into nucleus and cytoplasm, both are necessarily 

 related to nuclear division. 



In the unicellular Protista, in which each individual has a single 

 nucleus, or a pair of complementary nuclei, division of the nucleus 

 entails division of the whole organism : mitosis is an act of 

 reproduction. In multicellular organisms, division is merely a 

 concomitant of growth. In these under suitable conditions such 

 growth, like reproduction in the Protista, may continue indefinitely. 

 Such conditions occur in the higher plants where vegetative pro- 

 pagation can be carried on without limit, either naturally in the 

 formation of bulbs, runners, viviparous shoots or apomictic seeds 

 (to which special consideration will be given later) or artificially in 

 propagation by cuttings and layers, buds and grafts. Many species 

 of plants and animals depend for their preservation on these purely 

 asexual or " vegetative " means of propagation. This is usually 

 associated with great uniformity within the species, which in plants 

 may be described as " clones." 



Reproduction by mitosis alone can therefore continue indefinitely. 

 Cases where it has failed to do so and where vitality can then be 

 restored by sexual reproduction are due to special adaptation to 

 conditions of sexual reproduction. 



(ii) Sexual Reproduction. Sexual reproduction is known in all 

 groups of living organisms with nuclei. In the Flagellata it has 

 only been established in three genera {cf. M. Robertson, 1929) and 

 in the Protista as a whole it is difficult as yet to estimate its 

 importance. Elsewhere, however, whether normally or in some 



