28 



BOTANY OF THE LIVING PLANT 



involve a readjustment of the cells among themselves by a sliding 

 process, which is specially obvious where the cells become elongated 

 or very wide when mature. They seem then to push the surrounding 

 cells aside. Special names used to be applied to all the different 

 characteristic forms of cells. But this is not necessary except in 

 extreme cases ; thus the old name parenchyma is kept for a tissue of 

 roughly spherical or oblong cells with square ends, while long thick- 

 walled cells with pointed ends are c2^\Qd fibres (Fig. i6, i.-v.). 



(2) Changes in the Thickness of the Wall : Pits. 

 During the growth of the cell the wall is stretched, and like a 

 rubber sheet it would become thinner as it yields, were it not for the 



deposit of new cell-wall substance by the 

 cytoplasm. In ordinary cell-walls this is 

 effected by apposition of successive layers 

 upon the surface of the wall, and so 

 quickly that the stretching wall, even of 

 an enlarging cell, actually grows thicker. 

 In the mature cells with thick walls the 

 layers of stratification can often be clearly 

 seen (Fig. 17). But the thickening of the 

 walls is seldom uniform. Certain areas 

 are left thin, and a noteworthy feature is 

 that the thin areas in adjoining cells 

 usually correspond. Such areas are called 

 Pits, and the thin partitioning wall is 

 called the pit-membrane. Pits are of use 

 in facilitating the physiological communi- 

 cation between cells, and practically all mature cells show pits of 

 some sort on their walls (Figs. 17, 18 B, 19 B). 



Fig. 17. 



Section of a cell of Hoya carnosa, 

 with greatly thickened, stratified, 

 and pitted walls. The pits are very 

 narrow, and often branched. (After 

 von Mohl.) 



(3) Changes of Substance of the Cell-walls. 



In the young state the cell-walls are composed of a carbohydrate 

 substance, cellulose {C^Yi-^^fl.^^,, with more or less of those pectin- 

 substances which form the basis of fruit-jellies. But as they become 

 mature the chemical and physical nature of the walls may change. 

 Some walls becom- lignified or woody ^ and are then mechanically more 

 resistant and harder. Others become suberised, or corky, and are 

 then impervious to the passage of fluids. Others may become gummy, 

 or mucilaginous, and are liable to swell greatly on access of water, 



