THE AGGREGATIONS OF CELLS. 6? 



are formed. These may extend in any direction, but they 

 generally run parallel to the axis of that part of the plant in 

 which they are found. Other cell-fusions give rise to irreg- 

 ular branching tubes, or they may even form an extended 

 network (e.g. , in the laticif erous tissue of Cichoriaceae, Fig. 

 65, p. 75). 



95. (4.) Tissues. A tissue may be defined as an aggre- 

 gation of similar cells (or cell-derivatives) connately united. 

 There are three conditions of aggregation : 



(a) Cell-rows. In these the cells are united by their ends 

 into a row or filament. Such simple tissues result from cell- 

 fission in one direction only. In some cases, as in Oscilla- 



Fig. 51. Succulent parenchyma from the stem of Indian corn ; transverse flection. 

 yw, simple p)ate of cellulose, forming the partition-wall between two cells; 2,2, 

 intercellular spaces caused by splitting of the walls during rapid growth. X 550. 

 After Sachs. 



toria, the cells are short and broad, while in others e.g., 

 Spiroyyra, Zygnema, and the hyphae of many fungi they 

 are cylindrical or greatly elongated. Numerous cases occur 

 in the higher plants, the most familiar being jointed hairs. 



(b) Cell-surfaces are composed of a single layer of cells. 

 They result from cell-fission in two directions. Examples 

 may be found in many Ulvaceae, and in the leaves of somo 

 Bryophytes. 



(c) Masses. Where the cell-fission has been in three di- 

 rections the result is a mass of greater or less solidity. Fre- 

 quently, through cell-fusions, the elements which compose 

 such masses are cell-derivatives, instead of cells ; these may 

 be regarded as tissues of a higher order. 



