THE BOTANY OF THE FAII.M. 433 



PRACTICAL INFKIIKNVES. It will be obvious, then, from what has been 

 thus briefly sail, that for cultural purposes, such as the various operations 

 connected with tillage, the nature, quantity, and time of application of 

 manur". and the like, the character of root action in general, must be 

 studie , in connection with the nature and properties of the soil. The 

 ial form and characteristics of the root in the particular crop it is 

 wished to cultivate tap-rooted, fibrous-rooted, fleshy, surface-rooting, 

 or deep-rooting, etc. must also be taken into consideration in the same 

 relation. 



ION OF HEAT AND MOISTURE UPON LEAVES. A few words upon the 

 influence of excessive temperatures may be appropriately given. 



If the temperature fall below a given point, variable for each species 



and also for each individual plant, the functions of the leaf are held in 



\ ance, chlorophyll is only imperfectly formed (hence the yellow tinge 



of frost'-d wheat); and if the temperature be still further depressed, death 



; i ts. 



A TI< >N OF FROST. When a leaf is frozen, the fluid contents escape from 



the cells by permeation through their membrane, and freeze on the outside 



of the cell, so that the spaces between them are full of ice. It rarely hap- 



\a that the juices of the cells freeze in the interior of the cells if 



\ do, rupture of the cell wall and death are the most probable re- 



(Jnder ordinary circumstances, the cells lose that turgesc* 



which i> necessary for their activity. All the functions of life are arrested, 



not ily never to be resumed ; for, in some cases, when the ice in the 



r' the plant melts, tin- water is re-absorbed by the membrane, and 



lif- is resumed.* \V inter wheat must frequently become frozen in 



manner, but it is comparatively rarely that the plant is killed out- 



ra wisrly choo>in;4 those varieties which experience has shown 



hardiest. If the cold is sufficient to kill the leaves, or any por- 



them, the leaves become limp and Muekened. The limpin 



iy accounted i',,r l.\- the causes we have mentioned, as well as by the 



:; of supplies of water from tin- root. The discolouration i- the 



ie molecular change in the chlorophyll, at present nt under- 



8 to 



"N OF 1 Hi :\T ON LEAVES. Too high a temperature also 



or perverts all the functions of the leaf. When- transpiration is 



re, and the absorption of fresh supplies not in proportion, the 



lily wither, as may be seen in a field of mangels on a hot day, 



'ie evaporation of watery vapour from the surface is greater than 



f moUtuiv liy the root. On the other hand, during the 



BB 



