92 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 21. 



This definition may be further illustrated by the consideration of aggregates 

 of cells (or of coenocytes) which do not constitute true tissue. For instance, 

 the body of Hydrodictyon consists of a number of coherent coenocytes which 

 were originally free cells, and which are physiologically independent ; similarly 

 the cells forming the body of a Spirogyra do not constitute a true tissue, on 

 account of their physiological independence. In the higher Fungi (e.g. Agari- 

 cus) a nearer approach to a true tissue is made by the aggregation of filaments 

 (hyphce) of the mycelial body into masses such that the growing-points of the 

 nggregated hyphae form a collective apical growing-point. These various forms 

 of cell-aggregation may be distinguished as spurious tissues. 



The tissue, true or spurious, of which the body of a plant con- 

 sists may be either homogeneous or heterogeneous ; that is, the cells 

 (or the coenocytes) may be all alike, constituting therefore but one 

 kind of tissue ; or they may not be all alike, the different kinds of 

 cells being more or less grouped together so as to form different 

 kinds of tissue. A body which consists of different kinds of tissues 

 is said to be histologically differentiated. 



The structural differences between the various forms of tissue 

 in a histologically differentiated body are essentially connected 

 with the special adaptation of each form of tissue to the per- 

 formance of some particular function in the economy ; their differ- 

 ences are, in fact, evidence of their physiological interdependence. 

 In a histologically undifferentiated body (e.g., Hydrodictyon, 

 Spirogyra, etc.), the cells (or cosnocytes) are all similar because 

 they all have to discharge similar functions ; in fact, each cell 

 (or crenocyte) discharges all the functions of a living body ; at 

 first nutritive in function, it closes its existence as a reproduc- 

 tive organ. A body such as this, consisting of physiologically 

 independent structural units, is distinguished as a ccenobium. 



It is a remarkable fact that, whilst the cells of the various 

 tissues of a histologically differentiated body present characteristic 

 peculiarities of form, size, and relative arrangement, the most 

 striking distinctive peculiarities are exhibited, not, as in animals, 

 by the protoplasm of the cells, but by the cell- walls in respect of 

 their thickness, their chemical composition and physical properties, 

 and their markings (p. 103). 



Inasmuch as the cellular plants are the more numerous, and 

 present greater variety of structure, the following account deals 

 almost exclusively with them. And since the cell is the structural 

 unit of these plants, it will be advantageous to study the cell as 

 such first, and then to proceed to the study of the tissues. 



