The Living Materials 55 



ditions are right, grow normally. They main- 

 tain for the most part their normal morphology, 

 branching and extending in a regular way. The 

 details of this morphology are characteristic for 

 different species (Figs. 13, 21-24). There is, 

 for example, a very marked and wide difference 

 in degree of apical dominance (White, 1938, 66). 

 In the mustard, Brassica nigra, apical dominance 

 is extremely weak; as fast as branches are laid 

 down, they grow with a vigor equal to that of the 

 parent root so that an open network of rootlets 

 is formed without definite outlines. Roots of 

 clover, Trifolium repens (Fig. 13), on the other 

 hand have a very marked apical dominance so 

 that, even though many branches may be laid 

 down, their increment is greatly retarded as com- 

 pared with that of the original root and the 

 growth habit is that of a well denned and very 

 attenuate cone. The habits are thus roughly com- 

 parable to those of the white oak (Quercus alba), 

 on the one hand, and the white fir (Abies lasio- 

 carpa), on the other. 



Such roots can be excised with a minimum of 

 injury since the cross sectional area is small in 

 proportion to the volume of even a comparatively 

 short piece. They are meristematic for a consid- 

 erable length and consequently can be counted on 

 to grow with fair rapidity. They are naturally 



