36 THE WHEAT PLANT 



arise on the basal portions of the coleorhiza soon after the primary root 

 has broken through it. 



The seminal roots are all thin and of nearly uniform diameter through- 

 out their length ; when turgid and growing they measure from -5 to -75 

 mm. across ; later, when the soft cortical tissues are shrivelled, they may 

 become less than half this thickness. 



Numbers of very delicate lateral branches, often not more than a 

 tenth of a millimetre in diameter, appear on the rootlets, generally as soon 

 as these have reached a length of 4 to 6 inches. 



The seminal roots form but a small proportion of the total root-system 

 of the wheat plant, their function being concerned with the absorption of 

 the water necessary for the growth of the young plant, especially in its 

 early stages of development, although they appear to be functional through- 

 out the life of the plant, for they remain undecayed up to near the time of 

 harvest. The extent of their development and the depth to which they 

 descend is influenced by the texture of the soil and the depth at which 

 the grain is sown. Where the latter is deposited near the surface, the 

 seminal roots reach their maximum development and penetrate to a 

 depth of 8-12 inches or more ; where the grain is sown at greater depths 

 they grow feebly, the reserves of the endosperm being chiefly utilised in 

 enabling the shoot to grow and reach the light above ground (Fig. 60). 



The adventitious roots arise just near the surface of the ground from 

 the nodes of the main axis and its branches, their position being correlated 

 with the arrangement of the buds and leaves of the plant. The first to 

 appear is a pair of roots which grow out from the tillering node of the 

 primary axis (Figs. 22 and 23), one of them on the right, the other on the 

 left of the first lateral bud, their points of origin being not quite on the 

 same level. One of them appears before the other, and in its rapid growth 

 tends to push aside the adjacent bud from the initial position ; especially 

 does this happen when the opposite root fails to develop. 



At the second node another pair develops in like manner, and from 

 each of the succeeding somewhat thicker nodes, between which the inter- 

 nodes remain short, two roots appear arranged similarly in regard to the 

 nodal bud together with one or more on the opposite side to that on which 

 the bud arises. Of each pair, the first root to develop is always found on 

 the same orthostich of the stem. 



At the base of the straws near the ground, where the internodes are 

 longer and the nodes, therefore, not so crowded together, four or six roots 

 are produced at each node, i.e. two or three pairs symmetrically arranged 

 as in Fig. 24 ; the upper series of these usually form very strong oblique 

 props which assist in keeping the straw erect. 



Longitudinal planes through the points of origin of opposite adventi- 

 tious roots of the primary axis cut the plane through the buds and midribs 



