1854. 
243 
Vegetable Physiology—Weeds. 
Editors Oountry Gentleman —Perhaps there is 
no other item of expense and labor more considerable, 
in farming operations, besides the actual cultivation 
and securing-of crops, than that of killing weeds, those 
scourges of the finest farming sections of our country— 
hence something said upon the subject is .always in 
place, even if not new. 
There are but three modes practical, for their ex¬ 
termination, frequent cutting, rooting them out, either 
by the hand or by the plow, or similar instruments, or 
choking them out with more luxuriant crops. The 
mode employed must of course be decided by the cir¬ 
cumstances of the case. 
All plants, (save the lowest orders and some parasi¬ 
tic plants,) draw their subsistence, their food, from two 
sources, by means of two very different sets of organs : 
from the soil by means of the roots, and from the air 
by means of the leaves. Both of these are essential 
to their well-being, the sap circulating from the one to 
the other, and any attempt made to kill the plant must 
be aimed at one or both of these organs. The roots 
absorb water, and in it are dissolved the various ma¬ 
terials found in the soil, the same that are used in 
building up the vegetable structure, but this sap is not 
yet ready to perform its ultimate office. It? must un¬ 
dergo certain chemical changes, and these must take 
place in the leaves, under the influence of light and 
air. A portion of the water is evaporated, the sap be¬ 
comes thicker, and. new substances are formed by the 
addition of new materials absorbed by the leaves from 
the surrounding atmosphere, principally carbon from 
carbonic acid, and nitrogen from ammonia and per¬ 
haps other sources. This sap, thus changed, is ready 
to perform its office, and nourishes the plant, adding to 
its growth as it passes down again toward the ropts? 
Every one has noticed the change in the sap of the 
maple or other trees when the leaves begin to grow, in 
the spring. In many plants, such as the trees and ma¬ 
ny herbs, the sap peases up through the wood, (the 
sap-wood of trees,) to the leaves, is then changed, and 
passes back towards the roots between the wood and 
the bark. -And here the increase of diameter takes 
place, every year adding a new ring of wood. In oth¬ 
er plants, such as canes, grasses, palms, &c., the sap 
passes both up and back in the interior of the stalk, 
and here also the increase of diameter takes place. 
Such plants are destitute of a true bark. The differ¬ 
ence of the modes of growth characterizes the two great 
natural divisions of flowering plants, as described by 
botanists, but in both eases the relative uses of these 
organs are the same. The leaves, to perform their 
peculiar office, have many openings or pores ( stomata) 
in their surfaces, to absorb the necessary gases, and 
also the dew and atmospheric moisture, and they pre¬ 
sent a very large surface by their immense numbers, 
opening their myriad mouths or pores to catch the de¬ 
sired materials from every passing breeze, purifying 
our atmosphere at the same time that they minister to 
their own wants. To perform their office the leaves 
must be green; they refuse to work when any other 
color. The colored leaves of autumn, the blanched 
leaves of plants growing in darkness, and the colored 
petals of flowers, will not elaborate sap and fit it for 
use of the vegetable economy. We can judge of the 
luxuriance of any crop by the relative shade of green 
it assumes. From their relation to the air leaves 
have been compared to the lungs of animals, and call¬ 
ed the respiratory organs of plants; but the analogy 
does not hold^ood in many important particulars, yet 
they are no less essential to their economy, and we 
will always kill a plant if we keep it entirely deprived 
of leaves for any considerable length of time. 
The ultimate aim of the plant seems to be the per¬ 
fection of its seeds, and to this end it puts forth all its 
energies. It is in most vigorous growth at the time of 
flowering, and many species if they be cut at this time, 
may be effectually killed. They appear not to have 
strength to put forth fresh leaves. This in a measure 
appears to be the case with the Canada thistle. I 
have known them nearly exterminated by a single 
cutting, when they were in the proper stage, that is, 
when some of the earliest heads were just in flower, 
but the majority not yet expanded; and this opera¬ 
tion if performed several times is sure to kill them— 
the only impracticable point is to cut them all and at 
the right time. The season, too, appears to make some 
difference, they being easier killed some years than in 
others. I have known a man to sell “ recipes for the 
sure killing of Canada thistles,” for 810 each, in a 
district troubled much with these pests, and his secret 
consisted in cutting them twice each year, on the '20th 
of June and 20th of September. This would kill them 
entirely, if effectually performed, if not in one year, 
in two or three. The time should be varied, however, 
with the season, some being so much earlier than oth¬ 
ers. All weeds that can be cut, that can not be got at 
easily in any other way, should be cut two or three 
times each summer, and there will soon be left few to 
cut. 
But there are many to which this is not applicable, 
from various circumstances—their size, habits, the 
grains in which they occur, &c. Such is the case with 
such as trail on the ground, some that commence flow 
ering very early in the spring, some that are not killed 
short of cutting a great number of times, as is the case 
with some troublesome grasses, and others that grow 
only in certain grains, and are not easily distinguished 
and separated from them. To these we must apply one 
of the other methods, rooting them out, or choking them 
out. Some may be easily exterminated before they 
become too numerous, by pulling them by hand, and 
every farmer should be in the habit of plucking up 
such occasional plants of suspicious aspect as he may 
meet with on his farm, even if he does not know that 
it is a troublesomeweed,” for some only become so 
when very numerous. Happy would it be if every 
farmer was a botanist, to quickly determine and recog¬ 
nize all unusual plants, and a vegetable physiologist, 
to use the best means for their extermination, if such 
were desirable. 
