26 SOIL CONDITIONS AND PLANT GROWTH 



smaller weeds, and effectually prevents their growth, " smothering 

 them," in the language of the farmer. This is often the cheapest way 

 of cleaning weedy land. A newly-mown lawn is yellowish if the grass 

 has been allowed to grow rather long, while the interior of a compact 

 tree like the beech is leafless. Forestry practices afford other illustra- 

 tions : young woods are planted densely in order that the stems of 

 the trees may be kept free from branches and the timber free from 

 knots ; later on, however, more light is desirable ; heavy thinning of an 

 oak, or beech, forest a few years before the final felling much increases 

 the amount of growth. F. C. Schiibeler (253) maintained that the ex- 

 tension of the hours of daylight during summer in northern latitudes 

 more than counterbalanced the low temperatures, and actually shortened 

 the time between sowing and harvest ; Wille, however (309), has criti- 

 cally examined the evidence and finds nothing to support this view, all 

 observed differences being readily explained by differences in variety 

 of crop, or in local conditions of soil and climate. 



Many years ago Siemens (265) pointed out the advantages of 

 artificial light for greenhouse work, but no method has yet come into 

 practice. 



Temperature. Fig. ib shows the general relationship between 

 temperature and plant growth. The gradient of the curve is at first 

 very steep, a slight temperature increase producing a marked increase 

 of growth ; above a certain temperature (which varies somewhat with 

 the conditions) the rate of growth falls off ; at higher temperatures 

 the plant suffers, the various processes no longer work harmoniously, 

 and the protoplasm loses efficiency till finally the plant dies. 



For purposes of crop production the temperature range is limited by 

 certain secondary effects. If the temperature is too low a purplish pig- 

 ment appears in the leaf, and the plant grows so very slowly that it is 

 liable in its early stages to succumb to insect pests, such as wireworms, 

 and in its later stages to be cut down by autumn frosts before it has 

 had time to ripen ; if, on the other hand, the temperature is too high, 

 the plant becomes taller than usual, less robust and, when much 

 water is also supplied, liable to all the fungoid pests that give so 

 much trouble in commercial greenhouses. Only over a comparatively 

 restricted range of temperature is it possible to obtain the compact 

 sturdy habit aimed at by the grower. This favourable range has not 

 as yet been correlated with other properties of the plant and has to be 

 discovered empirically ; it is, on the whole, lower for the seedling than 

 for the growing plant, but it is highest for the period of maturation. It 

 varies for different crops : wheat requires a cool time for sowing but a 

 hot time for ripening, barley requires a cool and oats a still cooler time 



