Development of Roots of Agricultural Plants. 
433 
sion requires {pro re nata). In fruit-trees it is quite certain that 
the depogit is often made in the bark and branches, and hence 
the necessity of rest, and the notorious fact that many trees will 
bear a crop only in alternate years. 
Liebig considers that the object of the winter rest in wheat is to 
give time for storing up nutriment in the roots for the formation of 
the stalk and ultimately of the seeds in the fructifying stage. 
" A very mild autumn or winter," he says, " operates unfa- 
vourably on the future crop, as the higher temperature encourages 
the development of the principal stalk before the proper time, 
and consumes the food which should have served to form buds 
and new roots, or to increase the store of organisable matter in 
the roots. Thus stunted in its development, the root supplies 
less food to the plant in spring, as it takes up and gives out less 
in proportion to its smaller absorbent surface and more limited 
supply stored up in it, and it retains the same feeble character 
in the succeeding periods of vegetation. The agriculturist 
endeavours to meet the difficulty by grazing down or cutting 
these feeble plants ; the formation of buds* and roots hereupon 
begins anew, and if the external conditions are favourable, and 
the plant has time to fill the root with a fresh store of organisable 
matter, the normal conditions of growth are, in the agricultural 
sense, restored." 
As regards cereals, the process of vegetation goes on con- 
tinuously in most varieties ; but in winter-wheats there is often a 
period of rest. The circumstance here, however, is rather acci- 
dental than essential, and dependent on their requiring a longer 
time to come to maturity. Our author's reasoning on this 
matter seems to partake too much of theory. Excellent crops 
are often produced where there has been no winter rest, pro- 
vided the growth is not so rank as to prevent proper ventilation ; 
and one object at least of feeding off is to obviate an excessive 
development of tillers, which are sure, in the end, to be com- 
paratively unproductive, either from the layering of the crop, or 
the want of space for healthy gi'owth, which cannot take place 
without due admission of light and air. 
There is a circumstance about most leguminous plants which 
has not yet been sufficientlv studied. From the earliest period 
of growth, little tuberiform bodies are formed upon the roots, 
which have been described as Fungi. They sometimes ac- 
quire a considerable size, as in the common acacia. Like the 
secondary roots themselves, of which they appear to be modifica- 
tions, they are derived from within and burst through the outer 
coat, soon becoming much inflated, and often conugated. Their 
walls arc thick and fleshy. The lower part of the axis consists 
of elongated, close-packed cells, while the swollen portion, at 
least in the kidney-bean, is filled, especially above, with tissue 
2 F 2 
