258 A TEXTBOOK OF BOTANY [Ch. V, 9 



biennials and herbaceous perennials, which store their food 

 perforce underground. Beets, Carrots, Turnips, Radishes, 

 and Sweet Potatoes are the most familiar examples. Not 

 all, however, of the farmer's "root crops" are roots, for 

 some are stems, as in potatoes, though this purely morpho- 

 logical distinction has no importance whatever in economics, 

 and very little in physiology. 



Because root-absorption is osmotic, and therefore requires 

 a soil solution much weaker than the sap of the root hairs, 

 it injures plants to over-fertilize them ; for fertilizers are solu- 

 ble, and thus increase the strength of the soil solution. But 

 the matter is also complicated by chemical relations, and 

 the stimulation given to growth of soil Fungi and other 

 organisms. 



Because roots need air for their respiration, wet or 

 clayey soils must be drained for our crops, often at great 

 trouble and expense. Yet water also is necessary, and must 

 be conserved for dry times. The art of drainage consists 

 in the maintenance of a beneficial balance between water 

 and air throughout the growing season. It is because roots 

 find this balance so excellent in drains, which therefore they 

 tend to fill, 'that gardeners must take care where they place 

 plants having specially hydrotropic roots (Willows, Poplars, 

 Elms). Where a tree happens to grow on ground which 

 must be graded to a higher level, a wall holding back the soil 

 from the trunk will often permit enough aeration of the 

 larger roots to save the tree, though frequently it does not. 



Like other parts, but perhaps more than they, roots need 

 warmth when growing. This is why hot beds are used in 

 the spring, — the heat being developed by the respiration of 

 organisms producing fermentation or decay in the manures 

 which are used. The same value inheres in " bottom heat" 

 supplied through pipes in the soil, sometimes used in forcing 

 greenhouse plants. 



Any osmotic process, which root absorption is, proceeds at 

 a rate directly proportional to temperature. Consequently 



