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 arid 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. Absorption. 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- 

 ings, 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 



