CELLS AND TISSUES zi 



numbers, as compared with the adjacent pericyclic and cortical 

 cells, that the term starch sheath has been applied to the layer. In 

 addition to starch, other substances such as tannin, mucilage, and 

 occasionally crystals may be present. 



The structure of the cell wall is the outstanding feature of this 

 tissue in cases where Casparian thickenings are laid down. The 

 endodermal cells are at first uniformly thin-walled and paren- 

 chymatous with no characteristics which might distinguish them 

 from adjacent cells. In the early development of the root, thicken- 

 ings are laid down on the radial and end walls which consist of 

 strips of wall material deposited on the primary wall. The strips 

 are suberized or cutinized and as seen in transection (in this view 

 referred to as Casparian dots), may appear to consist of a central 

 core of cellulose and pectic substances covered by a layer of suberin 

 or cutin. It is also possible that the water-proofing substances 

 may infiltrate into the primary wall to some degree; but, in any 

 event, the effect is to allow water and solutes to pass through the 

 protoplasts and tangential walls of the endodermal cells, and to 

 greatly restrict the movement through the radial and end walls. 

 The width of the strip is variable, being very narrow in some cases 

 so that in transection it appears as a rounded or oval dot; while 

 in others it may equal the width of the whole radial wall, which 

 then appears in transectional view to be more or less spindle- 

 shaped. At this stage, according to Eames and MacDaniels (15), 

 the tangential walls consist of relatively thin cellulose with few, 

 if any, pits. Simple pits which extend through the Casparian strip 

 when it is wide occur in the radial walls and there are some pits 

 in the end walls. 



The endodermis frequently remains in this secondary stage of 

 development without further modification; but there may be a 

 tertiary phase in which additional secondary wall thickening 

 occurs involving the inner tangential, radial, and end walls and 

 occasionally the outer tangential wall. These walls are sometimes 

 much thickened and lignified, the size of the cell cavity being very 

 greatly reduced. In roots where tertiary development occurs, cer- 

 tain cells usually opposite the protoxylem points may remain in the 

 secondary condition and are referred to as passage or transfusion 

 cells. In this stage, the endodermis has no further capacity for divi- 

 sion and growth; and, if the axis continues to enlarge, its cells are 

 ultimately ruptured, together with the structures outside it. 



