18 THE CELL 



cases to speak of "cell-colonies," "cell-families," or " coenobia," and 

 thus to attach the notion of individuality exclusively to the component 

 cells. The delicate cell-plates of Pediastrum, the sac-like net of Hydro- 

 dictyon and the cell-chains of Scenedesmus are all familiar examples of 

 such cell-colonies or coenobia. With every rise in the scale of evolu- 

 tion, on the other hand, and with every increase in the perfection of 

 organisation, the independence of the component cells becomes more 

 and more subordinated to the individuality of the plant as a whole. 

 Certain elements may indeed lose their individuality altogether. 

 T!i us cells or cell-masses frequently coalesce with one another, or 

 become disorganised in various ways. The " syncytes " (ZcU-fiisioncn) 

 produced in this way are employed for very diverse physiological pur- 

 poses, being utilised as water-conducting or as laticiferous vessels, as 

 sieve-tubes, as secretory cavities, and the like. That complete loss of 

 individuality on the part of the single elements is by no means incom- 

 patible with the performance of a physiological function, is further 

 demonstrated by the extensive use which is made of dead cells in the 

 construction of the plant-body. The living organism, whether vegetable 

 or animal in nature, has often been described as a " society of cells." 

 After what has been stated above it will be admitted that there is much 

 to be said in favour of this comparison. The simile is undoubtedly 

 justifiable so far as the main principle is concerned ; for in the 

 case of human society it is likewise necessary to discriminate between 

 the individuality of the state, on the one hand, and that of the citizen 

 who represents the social unit, on the other. 



B. THE STRUCTURE AND FUNCTIONS OF THE TYPICAL 



VEGETABLE CELL. 



The use of the term " cell " to denote the structural and functional 

 elements of which the bodies of plants and animals are composed, 

 recalls the historical fact that Robert Hooke, the discoverer of the 

 cellular structure of plants, was cognisant only of the relatively firm 

 cell-membrane or wall. 



The presence of a hard case of this kind enclosing the " living 

 contents," or protoplasmic cell-body, is a characteristic feature of the 

 typical vegetable cell, and distinguishes it from the cells of animals. 

 In the present chapter, however, the living protoplasm and the various 

 protoplasmic organs will be dealt with first; subsequently the cell- 

 wall will be discussed as far as its general features are concerned. 4 



1. General Characteristics of Protojihixm. 



In all cases, without exception, it is the protoplasm (v. Mohl), or 

 protoplast (I [iinstein), which must solely and exclusively be regarded as 



