12 



General Botany 



weakest link, so the development of a plant is limited by the 

 factor which is least favorable. 



Fig. 5. Differences in form of the water smartweed (Polygonum amphihium) due to the 

 environment: A, grown on moist soil, stem erect, covered with hairs; B, grown in water, 

 upper leaves floating, smooth throughout ; C, grown on dry soil, stems and leaves rough 

 hairy, and prostrate. {After Massart.) 



If soil conditions prevent the entrance of an adequate supply 

 of water into the roots, the stem and leaves will be dwarfed, no 

 matter how rich the supply of minerals may be. If the leaves 

 are exposed to unfavorable conditions of light or temperature, 

 their work will be retarded and the nourishment of the whole 

 plant curtailed even though water in abundance be supphed. 

 Corn grown in sand under the most favorable conditions of light, 

 temperature, and moisture is small and may fail entirely to 

 produce seed, because sand does not supply the needed minerals. 

 Or, if during the winter months corn is grown in a greenhouse 

 in the richest of soil, it attains only a small size, because the inten- 

 sity, or the duration, of light is insufficient for normal development. 



The practical farmer knows that the yield of a crop plant is 

 limited by the least favorable conditions of the environment. 



