58 



BOTANY 



PART 



Fig. 57 A). This band gives the appearance of a dark dot or a dark 

 lens-shaped body, Fig. 57 , Fig. 161 S) in transverse sections, while 

 it appears as an undulated band in radial longitudinal 

 section. In older endodermal cells, as in the cells of 

 the cutis, a secondary layer of corky substance is 

 deposited all over the wall, and thick tertiary layers 

 of carbohydrate material that often become strongly 

 lignified may follow on this. 



In the cutis tissue, when 

 this is a single layer, and 

 in the endodermis isolated 

 cells, characterised by their 

 and by their walls 



rrn 



not being corky, frequently 

 occur. These are known as 

 transfusion cells. 



(3) Cork. While the 



FIG. 57. A, Diagrammatic representation of a single endo- 

 dermal cell in the solid showing Caspary's band on the 

 radial walls. B, Endodermis in transverse section ; Gas- -> 

 pary's band appears as the dark lenticular regions of the 6 P 1Cl 



radial walls. tissue are alway s primary 



permanent tissues the 



cork is always a secondary tissue, and is developed from a secondary 

 meristem known as the CORK CAMBIUM. The cork forms either a 

 thin peripheral layer a number of cells thick which is smooth and 

 of a grey colour, or thicker fissured coverings of cork composed of 

 radial rows of cells (Figs. 58, 59). It forms where the epidermis 

 has been thrown off, or where living parenchyma has been exposed by 

 wounds. The cork cells usually contain air and are brown, owing to 

 the dead cell contents. They have a flattened prismatic form and are 

 extended tangentially, fitting together without intercellular spaces. 

 The secondary layers of the wall are suberised, while the middle 

 lamella is often lignified. Tertiary thickening layers are either 

 wanting or consist of cellulose forming the so-called cellulose layer 

 which may sometimes become lignified. Even a thin layer of cork 

 a few cells deep (Fig. 59) greatly diminishes the transpiration from 

 the surface of any part of the plant, and, as will readily be under- 

 stood, much more than the epidermis does. Thicker zones of cork 

 also prevent the entry of parasites. Since cork is a poor conductor 

 of heat it also protects the plant against over-heating. 



Many old stems, tubers, bud scales, and fruits are covered with a layer of cork ; 

 thus the skin of a potato is of this nature. Bottle-cork is obtained from the 

 Cork Oak. 



The mature cells of cork are very rarely pitted, and either remain relatively 

 thin (Fig. 58) or are more or less strongly thickened (Figs. 59, 185 p). Strongly 

 thickened cork cells form what is known as STONE CORK. The cells of cork may be 

 completely filled with dead contents (Fig. 59) which have usually a brown colour. 



Frequently layers of suberised and unsuberised cells alternate in a corky tissue. 

 The latter cells, which do not differ greatly from the cork cells in structure and 



