CHAPTER V 



THE DESCENDING SAP 



And now returning through the knotty stem 

 By broader routes, a copious, nutrient stream. 



WE saw in Chapter III. that an ordinary foliage leaf consists of three 

 distinct kinds of tissue, which may be popularly described as the 

 veins, the fleshy substance between the veins, and the thin enveloping skin. 

 Here is a microscopic view of the transverse section of part of a Rhodo- 

 dendron leaf (fig. 158). Upwards of twenty layers of cells are packed in the 

 thickness of this single leaf a fact to arrest attention. The double line of 

 cells lettered a belongs to the epidermis of the upper side of the leaf; 6 

 shows the fibre-vascular bundle of a vein in cross-section ; c the par- 

 enchyma of the ground tissue, 

 easily to be recognised in the 

 actual leaf by the chlorophyll 

 corpuscles contained in the cells ; 

 d are air cavities between the cells, 

 botanically known as intercellular 

 spaces and the single row of cells 



'li&WVf UfWQWWtffi h at the bottom of the section be- 



longs to the epidermis of the under 

 side of the leaf. 



But some cells have yet to be 

 spoken of which play a most im- 

 portant part in the life of the 

 plant, and to which particular at- 

 tention should be given. A pair 

 of these cells are lettered / in the 

 drawing. They project from the 

 lower line of epidermal cells, and 

 form the two lips of a little mouth, 

 which communicates with one of 

 the intercellular spaces (d) ; more- 

 over, they contain chlorophyll, 

 which the epidermal cells do not. 

 To speak of these projecting cells 



FIG. 158. SECTION THROUGH PART OF THE 

 LEAF OF A RHODODENDRON. 



124 



