THE aggheoations of cells. 



67 



■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.(/., in the laticiferous tissue of Gichoriacese, Fig. 

 65, p. 76). 



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 tlieir ends 

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

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



A 



Fig. 51. — Succulent parencliyma from the stem of Indian corn ; transverse pcction. 

 -^w^ simple plate of celluloee, forming the partition-wall between two celld ; z, s, 

 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., 

 Spirogyra, Zygnema, and the hyphee 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 Ulvacese, and in the leaves of some 

 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 ce}l-fusions, the elements which compose 

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

 be regarded as tissues of a higher order. 



