88 



HISTOLOGY AND PHYSIOLOGY. 



(Fig. 200), Nostoc (Fig. 201). A common example of a 

 cell-colony, or spurious tissue, is furnished by 

 the fresh-water Alga, Water-net (Hydrodictyon). 

 The separate motiie cells (zoospores) into which 

 the protoplasm breaks up arrange themselves 

 and unite so as to form a net while yet in the 

 mother-cell. The term tissue is used to denote 

 a permanent union of cells. They may be 

 united end to end, so as to form a row, or fila- 

 ment, as in the filamentous Algae (Spyrogyra, 

 Zygnema, Figs. 189, 190), jointed hyphse (or 

 200 vegetative threads of Fungi), and many hairs 



of the higher plants. When cell-fission takes place in two 

 directions, a cell-surface, or single layer of cells, is the 

 result. Examples are furnished by some Algse ( Ulva), and 

 the leaves of some of the Mosses. In ordinary tissue, cell- 

 fission takes place in three directions, resulting in a mass 

 of greater or less solidity. 



113, It sometimes happens that the partition wall be- 

 tween two adjacent cells becomes absorbed, and the two 

 cells fuse, as it were, into one. Thus by cell-fusion of many 

 cells, long tubes, or ducts may arise. Aside from cavities 

 so formed, and those of the individual cells, there are 

 others, called intercellular spaces. Rapidly growing 

 parenchymous tissue exhibits numerous small intercellular 

 spaces at the corners of the cells, which 

 liave resulted by the splitting of the walls 

 and the partial receding of the cells from 

 each other (Fig. 203, in). In like manner 

 continuous vessels are sometimes found, as 

 in the tissue of the Pine family, where they 



Fig. 200. Gleocapsa forming a cell-colony. Fig. 201. Nostoc^ a cell-colony. 



