THE LEAVES 



55 



a length of 300-400 /z, while on T. aegilopoides the sparsely distributed 

 long hairs on the surface of the leaf-blade measure i mm. or more. 



When present the trichomes are distributed in somewhat regular 

 order, at short intervals along the rows of epidermal cells on the margins 

 and surfaces of the blade. There are usually more on the upper than the 

 lower epidermis, and in T. vulgare the longest hairs are found in the row 

 of thick-walled cells which runs along the summit of each ridge ; shorter 

 ones are frequent on the flanks of the ribs, but few are seen among the 

 motor cells. 



Each stoma of the leaf-blade consists of four 

 cells, the two guard cells being narrow, with speci- 

 ally thickened walls round the stomatal pore and 

 thin-walled widely dilated ends : the pore when 

 closed appears as a narrow slit 30-40 /x long 



(Fig. 49)- 



The stomata are only very slightly sunk below 



the general level of their neighbouring cells and 

 run in single or double longitudinal lines, each 

 stoma alternating more or less regularly with 

 an elongated epidermal cell. On the upper sur- 

 face the rows on one side are contiguous with 

 the motor cells, and on the other side lie next 

 to several rows of long cells without stomata. 



In transverse section the pores of the stomata 

 are seen to be in communication with large 

 intercellular cavities in the mesophyll. The epi- 

 dermis of the lower surface of the leaf resembles 

 that of the upper surface in form and arrange- FlG 49 _ I? Stomaj sur . 

 ment of its elements, but there are no definitely face view; 2, median 

 marked ridges upon it, and motor cells are 

 absent : the cell walls are sinuous and much 

 thickened, especially in the lower half of the blade. 

 The hairs are also fewer in number, and the 

 stomata more often in single lines than on the upper surface. 



The ratio of the number of stomata on the upper and lower epidermis 

 respectively is usually about 10:7, the number found on the upper surface 

 in the few cases examined being about 7000 per square centimetre. 



The outer epidermis of the full-grown leaf-sheath is composed of rows 

 of elongated cells similar in form to those of the blade, but the walls are 

 more sinuous and stoutly thickened (Figs. 50, 52). 



The stomata are arranged in lines, as on the lamina, and a few hairs 

 are usually present. Over the thick nodal base of the sheath the epidermis 

 consists of much shorter cells, in the rows of which, at more or less 



transverse section ; 3, 

 transverse section across 

 one end ; 4, longitudinal 

 section of a guard cell 

 (x 3 8o). 



