6o 



THE WHEAT PLANT 



young ; in the nodal part of the leaf-sheath, however, the outer bundle- 

 sheath retains its chloroplasts for some time, accumulating later a large 

 number of fine round starch grains, each about 4 /u, in diameter. 



(d) Stereome of the Leaf. Above and below the bundles, and arranged 

 parallel with them along the leaf, are strands of stereome or supporting 

 tissue consisting of sclerenchymatous fibres, some of the walls of which 

 are so much thickened that the lamina of the cells are almost obliterated. 



In the case of a thin 

 intermediate bundle, small 

 isolated stereome strands, 

 consisting in some in- 

 stances of three or four cells 

 only, are present immedi- 

 ately within the upper and 

 lower epidermis above and 

 below the bundle, the space 

 between the latter and its 

 corresponding sclerenchy- 

 matous strands being 

 occupied by ordinary thin- 

 walled ground tissue. The 

 stereome above and below 

 a stout bundle is, however, 

 more extensive, usually fill- 

 ing up the space and form- 

 ing a strong girder between 

 the epidermis and the 

 sclerenchymatous bundle- 

 sheath with which it be- 

 comes united. 



In the leaf -sheath the 

 vascular bundles are gir- 

 dered to the outer epidermis 

 by strands of stereome 

 continuous with those of 

 the under side of the blade. 



On the inside of the sheath, which corresponds with the upper surface 

 of the blade, only a few cells of stereome are seen beneath the epidermis 

 opposite to the larger bundles but not connected with them as a girder ; 

 opposite to the smaller bundles none is developed. Traced downwards, 

 the stereome increases in amount, reaching a maximum in the thick nodal 

 portion of the sheath, where it forms a strong strand of semi-elliptical 

 section on the outside of each vascular bundle and free from the epidermis 



FIG. 57, i. Transverse section of the leaf-base just 

 above a node ( x 25). s, Stem; b, leaf-base; 

 r. stereome of bundle. 



FIG. 57, 2. Transverse section half an inch above i 



