I026 



A TEXTBOOK OF THEORETICAL BOTANY 



of the water supply and the channels of translocation of organic materials 

 from the leaf to other parts of the plant. 



The veins are always differentiated at the boundary between the palisade 

 and the spongy layer, and are hence practically median. Each vein is sur- 

 rounded by a sheath of elongated parenchyma, normally devoid of chlorophyll, 

 which is one cell thick in the finer veins, but increases in thickness as the 

 leaf base is approached, where the sheath cells pass into the massive parenchy- 

 matous coat of the midrib (Fig. 1016), which in turn is continuous with the 



n 



Fig. 1016. — Launis nobilis. Transverse section through the midrib 

 of a leaf showing a U-shaped vascular bundle surrounded by a 

 parenchymatous coat. 



ground tissue of the petiole. There appears to be a concentration of sugars 

 in these sheath cells, and they are sometimes called the glucose sheath. They 

 are probably important in translocation, especially at the vein extremities, 

 where the amount of vascular tissue in each vein is very small. The question 

 of an homology between these vein sheaths and the endodermis cannot be 

 definitely answered. A typical endodermis, with Casparian bands on the cell 

 walls, does occur in the leaves of several families, notably the Primulaceae, 

 the Plantaginaceae and the Rosaceae. At the leaf base it may even be 

 secondarily suberized, but among the smaller veins it is usually irregular 

 and incomplete, and the characteristic band on the cell walls is absent from 

 the smallest veins. In the majority of families an endodermis is altogether 

 absent from the leaf. 



The free end of each vein is formed of a single tracheid, which is often 

 enlarged (Fig. 1017). The phloem stops short of the end, and the last phloem 

 cells are peculiar, and may be regarded either as enlarged companion cells or 



