THE TWOFOLD NATURE OF CELLS 17 



the growth and differentiation of each organ and of the plant as a 

 whole is purely the result of formative processes which take place 

 within the individual cells. A reaction against this one-sided concep- 

 tion was instituted hy Hofmeister, who was the first to maintain that 

 the growth of individual cells is dependent upon that of the entire 

 organ of which they are the component parts. 



In accordance with its twofold nature as explained above a cell 

 plays now an active and now a passive part in the multifarious vital 

 activities of the plant. As a part of the whole it is subject to the 

 prevailing tendency of morphogenetic activity and growth ; as an ele- 

 mentary organism it exerts a certain amount of control over these very 

 processes, the extent of its influence depending upon its location and 

 upon its special characteristics. Consider, for example, a single one 

 among the innumerable green cells contained in an ordinary foliage 

 leaf. By virtue of its photosynthetic activity such a cell influences 

 the inception of new organs, and consequently the growth of the plant 

 as a whole. On the other hand, this green cell would be unable to 

 perform its characteristic functions, were not the leaf of which it is a 

 part first of all enabled by definite formative processes of growth to 

 assume its flatly expanded shape, and secondly induced by its specific 

 mode of reaction towards photic stimuli to take up the position in which 

 it is exposed to illumination in the most favourable manner. Since, 

 namely, every green cell requires a certain intensity of illumination in 

 connection with its photosynthetic activity, proper provision must be 

 made to meet this demand, before the cell is able to perform its normal 

 functions efficiently. On the one hand, therefore, the growth of the 

 plant as a whole depends upon the activity of the individual green 

 cells and upon the nature of the materials which they manufacture, 

 while, on the other, this photosynthetic function of the green cells is 

 itself influenced and controlled by the general growth of the plant and 

 by the character of the architectural plan embodied in its external 

 morphology. 



This interaction between the cells in their character of elementary 

 organisms on the one hand, and the plant-body as a whole on the 

 other, may naturally assume a variety of aspects ; the more marked 

 the individuality of one accent becomes, the more does the other become 

 subject to its control. Generally speaking, the independence of the 

 individual cells is most pronounced and effective in the case of plants 

 which stand at a relatively low level of evolution. Among the lowest 

 of the Green Algae, the Protococcales, the individuality of the plant- 

 body as a whole is so feebly developed, that the latter is not even 

 regarded as an individual in the sense in which this term is applied to 

 one of the Higher Plants; it is on the contrary customary in these 



B 



