INTRODUCTORY. 9 



functions of its life, in the higher forms there is a gradually 

 increasing distinction of the functions and a localisation of 

 them in certain parts of the cell, in other words, there are 

 indications of differentiation of function and of physiological 

 division of labour. 



In highly organised unicellular plants we also find indica- 

 tions of a differentiation of another sort, of a differentiation of 

 form or morphological differentiation. The simplest expres- 

 sion of this is the distinction of two regions in the body of 

 the plant, both lying in the line marking the direction of most 

 active growth, the base and the apex. In some forms this 

 morphological differentiation goes so far as to indicate that 

 marking-out of the plant-body into members, which, as we 

 shall soon learn, finds its full expression in the higher plants : 

 this is well seen in such forms as Caulerpa and Bryopsis. 



In the majority of instances plants consist of a number of 

 cells connected together. Some multicellular plants consist 

 of cells which appear to be all exactly alike, and, although 

 they are connected together, it seems that each cell, the proto- 

 plasm of which is usually highly differentiated, performs all 

 the functions of its life independently of the others ; this is 

 the case, for example, in the Confervaceae and Ulvaceae. The 

 functional or physiological differentiation of such multicellular 

 plants cannot, therefore, be considered to be higher than that 

 of a differentiated unicellular plant such as Haematococcus. 



In most multicellular plants, however, it is readily seen 

 that the constituent cells are by no means all alike, and 

 further that each different kind of cell is connected with the 

 performance of some particular function ; thus, in such a 

 plant, there are cells which are especially charged with the 

 absorption of the food, others in which the constructive meta- 

 bolism of the plant is especially performed, others again 

 which are reproductive in function, and so on. Further, it is 

 usually the case that cells of some one kind are characteristic 

 of some particular part of the plant ; that part is then said to 

 be the organ for the performance of the function for which 

 the cells in question are especially adapted, a term which is 

 also applied to any part of the protoplasm of a differentiated 



