288 THE BIOLOGY OF FLOWERING PLANTS 



another mechanical function fulfilled by roots and hypo- 

 cotyls. In rosette plants with a vertical root stock such as 

 the primrose, or in plants like the crocus, in which new 

 corms are formed yearly at a higher level than the old, 

 the new growth would soon project above the ground 

 if it were not pulled down. This is done by special " con- 

 tractile " roots (Fig. 42). They 

 are usually rather thick, and, after 

 becoming firmly fixed, they 

 undergo an active contraction by 

 the alteration in the shape of 

 definite tissues. The epiderm 

 does not contract, and comes to 

 have a curious wrinkled appear- 

 ance. This is well seen in the 

 roots of young crocus corms in 

 early summer. In some seed- 

 lings, too, a contraction of the 

 primary root and of the hypocotyl 

 brings the little plant well into 

 the soil ; this is particularly the 

 case in plants in which the 

 hypocotyl takes part in the 

 formation of storage organs, 

 which are thus pulled down to 

 a definite level. 



Fig. 42. — Contractile roots of 

 Chlorcea membrajiacea : the 

 ground level is indicated by 

 the dotted line ; the base of 

 the first root has been pulled 

 far down by the action of 

 the second. Nat. size. 

 (After Rimbach.) 



§ 3. Mechanical Features of 

 THE Stem 



The requirements of the 

 stem are very different. Did 

 the weight of the foHage and branches bear downwards 

 quite symmetrically the stem would be subjected to longi- 

 tudinal compression like a pillar. The stem is, however, 

 normally subject to continuous movement by the wind, and, 

 even in still weather, the bearing of the crown is never 

 exactly central. The result is that the stem is principally 



