558 Cultivation of Plants. 
are principally such as have hard woody or horny integuments, 
those of the Rose, for example. Experience has, moreover, 
taught that the older seeds are the longer they are in ger- 
minating. Some seeds must be sown almost immediately after 
they are harvested, as contact with the air causes them to 
decay and soon destroys their vitality; hence the difficulties 
encountered in introducing many desirable exotic plants. 
Others, again, will retain their germinating powers for a great 
number of years. And we may add that seeds buried too deep 
in the soil for atmospheric influences to reach them will pre- 
serve their vitality for a period to which we can assign no 
limits—perhaps thousands of years, as would appear from the 
plants that often spring up on newly moved soil and in fresh 
clearings, which are sometimes different from any previously 
seen in the surrounding country. 
2. dAbsorption.—This term is employed to designate the act 
by which a plant draws the materials necessary for its growth 
and sustenance from the soil and air. All parts of a plant 
contribute to the fulfilment of this function, or at least so long 
as they are young and herbaceous. But the root is the prin- 
cipal channel for the conveyance of the various constituents 
which go to build up a plant. And the tender extremities 
(spongioles) of their fibrils or ramifications are the most active 
points. Leaves, too, are provided with numerous minute open- 
invs, termed stomates, which, according to the state of the 
weather and the amount of moisture contained in the plant, are 
either open or closed. 
The elements taken up by plants through these two channels 
are either in the gaseous or liquid state, for not the minutest 
particles one could imagine to be held in suspension by 
water can enter. It may readily be conceived that very fine, 
almost impalpable grains of dust may mechanically pene- 
trate the stomates, but it does not follow that they are 
absorbed. On the contrary, they obstruct and destroy these 
passages and prevent the leaves from exercising their physio- 
logical functions in a regular manner, and consequently the 
health of the plant becomes impaired. This effect is well- 
known to gardeners, especially on window and conservatory 
plants, and on those in the open air near public roads, which 
they obviate by frequently syringing, or otherwise the plants 
would inevitably be choked. In the natural order of things the 
rains are sufficient to accomplish this purpose. The action 
