The Food of Plants. 349 
of nutrition. Science has repeatedly shown that an increase 
of sugar percentage in the beet, or of starch in the potato, 
is directly related to the supply of potash to the plant and 
the condition of availability in which that element is pre- 
sented, and the question has therefore more than once been 
asked,—is it not possible by a judicious control of the food 
supply, to bring about, more quickly, those changes which 
are known to have taken place between the wild and culti- 
vated plants, and in the latter to still farther improve their 
qualities? I think the results so far obtained justify us in 
answering this question in the affirmative, but before so 
doing, I must briefly refer to the relative value of nitrogen- 
ous and non-nitrogenous food substances in the two phases 
of growth through which all plants pass, namely, the purely 
vegetative, or that period during which mere extension of 
parts, as stem and leaves, takes place; and the reproductive, 
or that period in which the flowers are produced and the 
seed is formed for the growth of succeeding gener- 
ations. The elaborate series of investigations conducted by 
the Germans for many years, as well as the very notable 
investigations of Lawes and Gilbert at Rothamstead, Eng- 
land, in which continuous observations have been made 
upon various field crops grown on the same land and under 
the same conditions since 1835—all these results establish 
the general law that those foods in which nitrogen is in re- 
lative excess, promote the mere extension of structure and 
tend to retard the reproductive function. While on the 
other hand, those foods in which the mineral substances 
are in relative excess, tend to retard vegetation, induce an 
earlier maturity, and thus hasten the formation of seed, 
Probably many of you have observed how a plant fed with 
ammonia makes a most vigorous growth of leaf and branch, 
and acquires a deeper and richer hue, and how also, trees 
are similarly influenced when located in exceptionally rich 
places. A notable illustration of this was brought to my 
notice a few years since. The ground in a small peach 
orchard was utilised as a kitchen garden, and for this pur- 
pose annually received a heavy dressing with nitrogenous 
