THE STEMS 81 



Isolated plants grown with excess of nitrogenous manures frequently 

 possess strong rigid straw capable of effectually resisting the strain of wind 

 and rain ; they are, however, very liable to fall to the ground as a whole, 

 their anchoring adventitious roots, which in normally grown plants bind 

 the plant firmly to the soil and keep it upright, being soft and weak through 

 the slight development, reduced thickening, and imperfect lignification of 

 their mechanical tissues. 



In common with stems of all grasses, wheat straw contains a con- 

 siderable amount of silica, namely, from 2-3 to 4-6 per cent in its dry 

 matter or 55 to 70 per cent in the ash ; it occurs chiefly in the cell walls 

 of the epidermis, to which it imparts hardness. The strength and 

 elasticity of straw was formerly attributed to the presence of this substance, 

 and the application of soluble silicates to the soil was suggested by Way 

 and others as a means of combating the tendency to " lodging." Pierre, 

 however, discovered that the straw of laid wheat was as rich or richer 

 in silica than that of standing crops, and his researches showed that the 

 lower internodes of straws, on which the resistance to lodging chiefly 

 depends, were always poorer in silica than the upper parts of the stem 

 and leaves. 



The investigations of Lawes and Gilbert also demonstrated that a 

 high percentage of silica in the ash and dry matter of wheat straw is 

 associated with brittleness and weakness of the latter, and occurs in 

 seasons of bad harvests. Well-ripened straw of good seasons, possessing 

 strength and elasticity, contains a high amount of lignified tissues and a 

 correspondingly low proportion of silica. 



The parasitic fungi, Ophiobolus graminis and O. herpotrichus , invade 

 the lowest internode of the wheat plant, often so weakening or destroying 

 its tissues that the straw falls to the ground. 



Erysiphe graminis is also sometimes the cause of similar damage to 

 the wheat crop. Where the attack is severe, large numbers of plants 

 may go down ; the result, however, differs in appearance from true 

 examples of " lodging," as, in cases of injury by fungi, the individual 

 straws fall in all directions, whereas when laid by the wind or rain large 

 portions of the crop are bent over in the same direction. 



" SHOOTING " OF THE STEMS AND ESCAPE OF THE EAR 



Neither the primary axis nor any of the branches arising during the 

 tillering period continue to produce lateral vegetative shoots indefinitely, 

 for sooner or later inflorescences or ears originate at the apex of the 

 branches, after which lateral bud formation ceases in those particular 

 stems. 



By an examination of longitudinal sections of very young shoots in 



G 



