40 



BOTANY 



PAET I 



SECTION II 

 HISTOLOGY ( 3T ) 



THE CELLS AS ELEMENTARY UNITS OF THE BODY 



I. THE FORMATION OF TISSUES 



A. The Idea and Significance of Cellular Tissues 



Every close association of protoplasts enclosed in cell walls is 

 termed a tissue. 



Only the lowest organisms are composed of a single uninucleate or 

 multinucleate protoplast and are thus unicellular throughout their life. 

 Usually the body of a plant is multicellular, consisting of many 

 protoplasts separated by cell walls and thus forming a tissue. The 

 attainment of large size and more complex external organisation 

 is as a rule associated with such a structure. There are, it is true, 

 certain Algae (Siphoneae) which are externally highly organised, 

 while they consist internally of a single multinucleate protoplast. 

 These may be contrasted as non-cellular organisms with the ordinary 

 cellular plant, to the construction of which they form an exception. 

 The formation of a cellular tissue is of the greatest importance in the 

 development of more highly organised plants in enabling a division of 

 labour to be effected in the protoplasm of the body. The division of 

 the protoplasm into numerous protoplasts provides elementary parts 

 which can take over different duties. The cell walls separating the 



protoplasts isolate the latter more or less, 

 while at the same time increasing the 

 cohesion and the internal rigidity of the 

 whole body formed of the numerous soft 

 protoplasts. 



A very imperfect tissue formation is found 

 in those organisms the cells of which separate 

 from one another at each division, but remain 

 connected by the mucilage derived from the 

 FIG. 35. Gloeocapsa polydermatica. swollen cell walls. Such unions of more or less 

 A, ^Commencement ^of division; independent cells that have had a common origin 

 may be termed cell families or cell colonies. The 

 Schizophyceae, to which group Gloeocapsa (Fig. 35) 

 belongs, and the orders of the Volvocales and 

 Protococcales among the Green Algae afford numerous examples, and the descrip- 

 tions in the special part should be consulted. 



In the cell filaments and cell surfaces of those lower Algae in which the cells 

 are all equivalent but are united together, the characters of a definite tissue begin 

 to make their appearance. With the increasing number of cells composing the 

 organism we get a contrast between base and apex and the appearance of a growing 

 point, and also progressive division of labour among the cells. 



B, (to the left) shortly after divi- 

 sion ; C, a resting stage. ( x 540. 

 After STRASBURGER.) 



