SENSITIVENESS. 57 



it often must be in nature, by insects or other means 

 the secondary roots, instead of retaining their oblique 

 direction, assume that previously taken by the injured root 

 and pass downwards. 



The Action of Light and Heat on Boots. The direct 

 action of light upon ordinary roots is, of course, usually of 

 a negative character. The form and direction of growth in 

 the root may, however, be affected by differences of tem- 

 perature, experienced now on one side, now on another. 

 Darwin has shown that the movements of roots, due to 

 irritation or contact, are checked by too high or too low 

 a temperature. During their passage through the soil, 

 the roots must be constantly subjected to variations of 

 temperature, first on one side and then on another, these 

 variations giving rise to some of the curvatures and bends 

 of the rootlets. The effect of an excessive amount of heat 

 in the soil upon the germination of seedlings has been 

 studied by M. Prillieux, and is of interest as indicating 

 the conditions under which tuberous roots and root stocks 

 may, under certain circumstances, be formed. When 

 seedlings of French beans and vegetable marrows were 

 grown in an overheated soil, the caulicle or portion of 

 the stem above the root and between it and the seed- 

 leaves became preternaturally swollen and tuberous, while 

 growth in height was arrested. The increased development 

 arising from the heated soil took place, therefore, in the 

 very same organs which constitute the so-called " bulbs " 

 of turnips or " roots " of swedes or mangels. The in- 

 creased volume is due principally to an excessive develop- 

 ment of existing cells rather than to a multiplication of 

 new ones. 



