314 MANUAL OF BOTA^fY 



than broad. They exist in sheets or bands, usually one cell 

 thick. They include the cambium and phellogens of stems and 

 I'oots. 



When cells are formed by cell- division in a growing point, the 

 new cell-wall arises at right angles to the walls with which its 

 edges are in contact. Hence at their first formation all such 

 cells are cubical or nearly so. By growth in various direc- 

 tions, mutual pressure, &c., they change their shape and relative 

 dimensions, giving rise to collections having very different 

 appearances. 



The cell-wall, at first thin and composed mainly of cellulose, 

 thickens as already described, and becomes chemically altered, 

 giving rise to still more complicated structures. 



In the early changes that take place as permanent tissue is 

 replacing meristem, the original close contact of the cells becomes 

 frequently interrupted. During the changes of tension that 

 occur in a growing mass of cells, the walls are caused to split at 

 an angle where three or more are meeting, and so a small inter- 

 cellular space is formed. The several spaces in a tissue extend 

 till they communicate, and a system of intercellular channels is 

 formed which extends throughout the plant, containing generally 

 air. The intercellular space system so formed is very prominent 

 in certain parts, especially in leaves and in the stems of aquatic 

 plants {fig. 749). 



The changes that take place in the composition of the cell- 

 wall usually affect the primary septum between the cells in a 

 different way from the successive thickening layers. The origi- 

 nal wall, though altered in many important respects, can gene- 

 rally be distinguished in the thickened wall of stratified cells, 

 when it is known as the middle lamella {fig. 677, m). 



Paeenohyma. — A collection of cells, the individuals of which 

 remain more or less of equal diameter in all directions, is known 

 as xjarenchyvia. The cells may remain with thin walls, or the 

 latter may be thickened equally or unequally, giving rise to 

 various modifications, of which the following are the more 

 important : — 



a. Bound or Oval Parenchyma {fig. 672). — This is formed 

 of rounded or more or less oval cells, with small spaces between 

 them. It commonly occurs in succulent plants, and also in those 

 parts where the tissues are of a lax nature. It is connected by 

 various transitional forms with — 



6. Stellate Parenchyma, which consists of stellate cells 

 {figs. 620 and 673), or cells with an irregular outline produced 



