58 PLANT LIFE ON THE FARM. 



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 vege- 

 table 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 hight 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 increased volume is due principally to 

 an excessive development of existing cells rather than to 

 a multiplication of new ones. 



The Action of Moisture on Roots. Much more 

 obvious to the general observer is the action of moisture 

 on roots. The distance to which roots will travel in 

 search as it were of water, and the way in which luxuri- 

 ant growth and intricate ramification are promoted, when 

 access to it is obtained, are familiar facts. Too frequently 

 drain pipes get choked with a mass of roots whose 

 structure has been changed, and whose excessive growth 

 has been stimulated by the presence of copious supplies 

 of moisture. If there is an equal supply of water all 

 round, the growth of the roots will be uniform ; but if, 

 as is more often the case, there is more water on one side 

 than on the other, then the root will curve to the side 

 where there is the fullest supply, and the power thus 

 exerted to get at the water is greater than that of gravity. 



