34 THE WORK OF THE LEAF [chap. 



burning are similarly destructive or down-grade pro- 

 cesses, and just as the life of the animal is absolutely 

 dependent on the preliminary life of the plant to build 

 up its necessary food, so for our engine power we are 

 largely living upon irreplaceable capital stored up in 

 the coal and oil that were manufactured in earlier stages 

 of the world's history. 



The fact that the green leaf supplies the driving 

 power to the whole machinery of the plant finds a good 

 many applications in practice. It explains, for example, 

 how it is possible to kill out perennial weeds like thistles 

 or bearbine by persistent hoeing, which does not give 

 the leaves an opportunity of working, although the root 

 keeps throwing up fresh shoots from its latent buds. 

 Every time it does so, some of the material stored up in 

 the root is used to make the fresh shoot and get it up 

 above the ground ; if, however, the new leaves thus arising 

 are cut off before they have had time to manufacture 

 any surplus material and send it down to the root foi- 

 storage, the original stock is somewhat exhausted, until 

 by a repetition of the process the plant no longer 

 possesses any reserve material wherewith to lift a shoot 

 above ground. Persistence in the cutting-off process is, 

 however, necessary ; it is no good to spud a thistle once 

 during the year, because it possesses sufficient reserve 

 material in its root to get fresh leaves up to the light and 

 they soon repair the losses. 



Again, we see how unwise is the practice of many 

 gardeners who remove the leaves of tomatoes, vines, etc., 

 when the fruit is forming, with the idea of throwing all 

 the strength of the plant into the fruit or of letting in the 

 sun to ripen the plant. The removal of the leaves means 

 that the manufacturing processes of the plant are 

 stopped ; no more carbohydrates are formed, in these 

 cases no more sugar, and the fruits remain small and 



