698 LIFE : OUTLINES OF GENERAL BIOLOGY 



prompt growth and others that hinder. Two main results stand out: 

 first, that the structural character of a tissue may be much altered 

 by the environmental and nutritional conditions in which it is 

 cultured; but secondly, that the deeper physiological character of 

 the tissue, as shown by its reactions and modes of growth, may 

 remain strikingly persistent. 



PLANT TISSUES 



Since the cells of plants are surrounded by very definite cell-walls, 

 markedly contrasted with the deUcate cell-membranes in most 

 animals, the tissues are sharply defined, as is evident when we even 

 mention wood, bark, pith, and so forth. Yet on the whole the 

 division of labour is less pronounced than in animals above the 

 level of worms; and the absence of distinct muscular and nervous 

 tissues is enough in itself to indicate how wide is the parting of 

 the ways. 



Tissues almost always arise by the multiplication and side-by- 

 side apposition of cells. There are a few very unusual cases, such 

 as the feltwork in some Algae and Fungi, where tissue is produced 

 by an intertwining of long filaments. In the Early Devonian, before 

 there were any Ferns, there w^as a small race of unique land plants, 

 the Rhyniaceae, which show the beginning of vascular tissue, destined 

 to play, in Pteridophytes and Spermophytes, such an important 

 part in structural and functional advance, being especially dif- 

 ferentiated for the transport of water and food-stuffs and for support. 

 Vessels arise by the fusion of a row of cells, and may be illustrated 

 by wood-vessels and sieve-tubes. 



Another contrast between plant and animal tissue is that plant 

 cells are very frequently united by bridges of protoplasm which 

 traverse the cell-walls. This is indeed illustrated among animals, 

 but it is not characteristic. It must help in plants to unify the 

 whole body; and the Sensitive Plant and the Sundew afford signal 

 instances of the rapid propagation of a change from cell to cell over 

 a considerable distance. This must compensate in some measure for 

 the absence of nervous elements in the strict sense. 



When a cell divides into two, the halves of the new cell- wall may 

 be pulled asunder, leaving an interspace; and the continuation of 

 this process may result in the development of an intercellular 

 cavity, usually containing air and often of importance in the internal 

 aeration of the plant. Or the same result may be reached by the 

 splitting and recession of two adjacent cell-walls. Rather different 

 in origin are the intercellular spaces formed by the disintegration 

 or solution of cells; and these are oftenest used as reservoirs fot 

 water or for waste products. It has been maintained that the resin- 



