100 ON THE PHYSIOLOGY OF PLANTS. 



no way injured, as the plant depends upon them for its support ; 

 and if the spongelets be broken off, no nourishment can be obtained, 

 any more than we could if our mouths were fastened up. But before 

 leaving this subject, I must add a few words about planting seedlings 

 or any other plants by the dibble, which is of all the instruments 

 used in gardening the most dangerous ; it causes a hardness and 

 stiffness to the earth all round the plant for some weeks ; so that if 

 the plant survives at all, it is only by the greatest exertion on its own 

 part as well as that of the gardener. 



And now for those principal parts of a plant, the root and leaves. 

 By aid of the root alone it is that a tree or any plant is enabled to 

 keep its right position in the ground; andthejnore the head spreads, 

 so much the more do the roots ; so that, in fact, there is kept up a 

 kind of balance between the part above and the part below ground. 

 Again, it is from the roots alone that the plant looks for sustenance, 

 for it is not enabled to do as we or other animals can, run about to 

 seek our own wants ; but as it is stationary, so it depends upon the 

 roots and its spongelets for drawing to it all the nourishment that 

 can be found within its reach. Again, roots perform the office of 

 throwing out all the parts taken in by them from which no nourish- 

 ment can be derived. It is a singular fact, that they are just as 

 careful and anxious to avoid the light as the leaves and young shoots 

 are to turn to it. But the leaves are nearly of as much importance 

 to a plant as the roots, for until the bud has actually burst into leaf 

 no growth can take place ; and when the leaf is formed, it is there 

 that the sap is turned into pulp. In the dark, leaves, like our lungs, 

 take in oxygen from the air, and part with a portion of the carbonic 

 acid gas contained in the sap. In the light, the sap on the upper 

 surface of the leaf parts with the oxygen contained in the carbonic 

 acid gas, and as the oxygen goes off, the carbon remains, while the 

 sap, previously little less fluid than water, is converted into a sort of 

 pulp, a considerable portion of which consists of carbon. The high 

 importance of leaves becomes thus manifest ; and nothing will more 

 enfeeble a plant than taking off its leaves in the growing season, 

 though they are no longer necessary during the cessation of growth 

 in the winter. And we must bear in mind that their fall previous 

 to winter is not caused by cold, but in consequence of the vessels at 

 the root of the leaf-stalk becoming gradually rigid, so as to prevent 

 the rise of sap, or at least the return of pulp. 



