144 



BOTANY OF THE LIVING PLANT 



though mechanically effective, is not fully specialised for giving 

 strength. The cells do not offer rigid resistance to elongation, but 

 they themselves grow with the growth of the part they support, 

 offering a persistent though plastic resistance. The position of the 

 collenchyma is usually peripheral, closely below the epidermis. This 



gives it its full mechanical 

 effect. In fluted stems it is 

 usually massed at the project- 

 ing ridges, and between them 

 it is often interrupted by thin- 

 walled green tissue of the 

 cortex. (Fig. 103.) Function- 

 ally it takes an intermediate 

 place in the individual develop- 

 ment between the state depen- 

 dent upon turgor and the full 

 rigidity of the mature state. 

 The resistance which it offers 

 to elongation is one of the 

 chief factors in producing the 

 mutual tensions of tissues 

 above described in the succu- 

 lent elongating stem. 

 The Sclerenchyma is, however, the more important specific mechanical 

 tissue. Its effectiveness depends partly upon its own characteristics, 

 partly upon its distribution. It consists of cells with thickened 

 walls, which are usually but not always lignified. When mature its 

 protoplasts are no longer functionally active, and may be represented 

 only by vestigial remains. It is then practically a dead tissue. The 

 form of the cells varies. Sometimes they are cubical or oblong with 

 square ends, and may be isolated, or disposed in groups. Such stone- 

 cells, or sclereids, give a hard gritty texture to the parts where they occur, 

 as in the bark or pith of various woody plants. The gritty nature of 

 the fruit of the Pear is due to nests of such stone-cells. But 

 frequently the sclerotic cells are elongated, and variously branched, 

 as in the sclereids of the leaf of Tea, or unbranched with flattened 

 ends, as in the leaf of Hakea (Fig. 104). Such cells are commonly 

 isolated. They stiffen the parts where they occur by the resistance 

 which their thickened walls offer to compression. The form, structure, 

 and function of the sclereids of Hakea, which prop out the firm 

 epidermis and thus give the leaf its remarkable stiffness, may be 



Fig. 103. 



Flowering stem of Astrantia in transverse section. 

 { X 10.) The collenchyma is dotted. 



