258 A TEXTBOOK OF BOTANY [Ch. V, 9 



Ijiemiials and herbaceous perennials, which store their food 

 perforce underground. Beets, Carrots, Turnips, Radishes, 

 and Sweet Potatoes are the most famihar 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 phj'siology. 



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 anfl expense. Yet water also is necessary, and must 

 he 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 hap]3ens to grow on ground which 

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

 from the trunk will often jiermit enough aeration of the 

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



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

 warmth when growing. This is why hot l.teds are used in 

 the s])ring, — the lieat being developed by the respiration of 

 organisms producing fermentation or decay in the manures 

 which are used. The same value inheres in "l)ottom heat" 

 supplird througli pip(\s in the soil, sometimes used in forcing 

 greenhouse plants. 



Any osmotic ]irocess, which root absoi-ption is, jiroceeds at 

 a rate directly i)roportional to temperature. Consequently 



