MAXIMUM DIFFERENTIATION 173 



produced, which afterwards develops into an independent 

 plant or animal of the new generation. It is a truly remark- 

 able circumstance that what we may consider as the highest 

 form of the sexual process should make its appearance so 

 low down in the scale of life. 



The nutrition of Vaucheria is purely holophytic : its food 

 consists of a watery solution of mineral salts and of carbon 

 dioxide, the latter being split up, by the action of the chro- 

 matophores, into carbon and oxygen. 



Mucor and Vaucheria are examples of unicellular plants 

 which attain some complexity by elongation and branching. 

 The maximum differentiation attainable in this way by a 

 unicellular plant may be illustrated by a brief description of 

 a sea-weed belonging to the genus Caulerpa. 



Caulerpa (Fig. 40) is commonly found in rock-pools 

 between tide-marks, and has the form of a creeping stem 

 from which root-like fibres are given off downwards and 

 branched leaf-like organs upwards. These " leaves " may 

 attain a length of 30 cm. (i ft.) or more. So that, on a 

 superficial examination, Caulerpa appears to be as complex 

 an organism as a moss (compare Fig. 40 with Fig. 81, A). 

 But microscopical examination shows that the plant consists 

 of a single continuous mass of vacuolated protoplasm, 

 containing numerous nuclei and green chromatophores and 

 covered by a continuous cell-wall. Large and complicated 

 in form as it is, the whole plant is therefore nothing more 

 than a single branched cell, or, as it may be expressed, a 

 continuous mass of protoplasm in which no cellular structure 

 has appeared. 



