SECT. I 



MORPHOLOGY 



101 



that a tissue arises through the interweaving of tubular cells or cell 

 filaments (Fig. 109). In such cases, where the filaments are so closely 

 interwoven as to form a compact mass of cells, the tissue thus formed 

 has the same appearance as the tissues of higher plants (Fig. 110). 



The mutual interdependence of the cells of a tissue is manifested 

 both by the conjunction of their pits (Figs. 73, 77) and by the 

 general similarity of their wall thickenings. 



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 swollen cell walls. 

 An example of this has already been seen in the Alga Gloeocapsa 

 poli/dermatk'a (Fig. 1). Such unions of more or less independent cells 

 that have had a common origin may be termed cell families or cell 



colonies. The Schizophyceae, 

 to which group Gloeocapsa be- 

 longs, and the orders of the 

 Volvocales and Protococcales 

 among the Green Algae aflford 



Fifi. 100.— Longitudinal section of the stalk of 

 the fructification of Bo/e<Ms f(/MZw. (x 300.) 



Fio. 110. — Transverse section of the sclero- 

 tiwm ot Claviceps ^mrpurea. (x 300.) 



numerous examples, and the descriptions 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 con- 

 trast between base and apex and the appearance of a growing point, 

 and also progressive division of labour among the cells. 



Growing Point. — This is the place of the increase in number of 

 embryonic cells and of the early distinction of tissues. In Thallo- 

 phyta an apical cell occupies the growing point ; this at first, as is seen 

 in Cladophora glomerata (Fig. 6), differs little from the other cells of 

 the filament. It assumes more and more the character of an APICAL 

 CELL from which all the organs of the plant take their origin ; thus, 

 in the case of Cladostephus yerticillatus (Fig. 7), the many-celled main 



