THE ANGIOSPERMAE : ROOTS 801 



is usually narrow, but it may sometimes cover the whole radial wall. It is 

 due to an impregnation of the middle lamella with a substance resembling 

 lignin but of uncertain composition. It also contains fatty materials. This 

 band maintains the cohesion of the endodermal cells, so that no intercellular 

 spaces are formed between them and as it is impervious to water it entirely 

 prevents any capillary movement through the endodermal walls, either to 

 or from the stele. Water passage through this layer must therefore be osmotic. 

 The region of the band in the wall normally appears convoluted, but it is not 

 clear whether this is always a natural consequence of a difference in growth 

 rates or whether it is simply an appearance due to the release of strains in 

 sections cut for microscopic observation. The protoplasm adheres closely 

 to the Casparian band and does not leave it even when the cells are 

 plasmolyzed. 



Endodermal cells are somewhat elongated, in comparison with cortical 

 parenchyma, and the bands surrounding them form a continuous meshwork, 

 which gives the endodermis considerable mechanical strength. It is in effect 

 a physiological boundary which controls the lateral movement of water, and 

 perhaps more importantly, of solutes. From the point of view of the water 

 supply to the upper parts of the plant it is the functional absorbing surface 

 of the root, as it is here that the water enters the vascular system. Indeed 

 in many grass roots the cortical tissues are only short-lived and the endo- 

 dermis becomes the actual as well as the functional absorbing surface. 



The primary condition of the endodermis is often followed by further 

 changes. Firstly there is the deposition of a suberin layer over the inner 

 surfaces of the cells, and secondly a thick deposit of cellulose mixed with 

 lignin. This thickening is commonest in Monocotyledons and may occur 

 only on the inner and side walls (the C-type) or all round the cell (the 

 0-type). In a few cases this renders the endodermis completely impervious, 

 but in most cases certain unthickened cells remain, directly opposite the 

 protoxylems, which are called passage cells (Fig. 796). Through these 

 some interchange of materials is kept up even in the older roots. 



The typical endodermis is especially characteristic of roots, but its 

 development seems to be determined to some extent by external factors. 

 It is usually most thickened in plants growing in places where the water 

 supply is scanty or variable, where its mechanical protective strength may be 

 valuable. It is rarely present in stems, except in some aquatic plants, in 

 which absorption may take place through the stems as well as the roots, and 

 it may be that it is only essential in absorbing organs, where its restrictive 

 nature assists the development of the water tension in the stele, by means 

 of which water is raised to the leaves. Little is known, however, about the 

 factors which determine its presence or absence. For example, it is absent 

 from the roots of some, but not all, aquatics and it may, on the other hand, 

 in some grasses, such as Festuca, be present in the stem and persist right up 

 into the leaves. There can be little doubt, however, that it is important chiefly 

 as a physiological limiting layer and that it is not simply the morphological 

 boundary of the stele. 



