I 



LECT. v ENEMIES OF CROPS AND TREES 77 



caused by a number of parasitical plants which 

 feed on the juices and ruin the crops. Even the 

 active agents of canker in apple trees and gum- 

 ming in plums are fungi, but the predisposing 

 cause of attack is ill-fed and immature growth. The 

 action of frost on this ruptures the tissues, and the 

 enemy takes possession by its spores. 



With healthy root action in soil which contains the 

 nutriment that trees need, and the thin disposal of 

 the branches, sound growth follows, and clean fruit- 

 ful wood. 



When trees are attacked by canker, pruning 

 and cleansing the branches, cutting back roots that 

 enter the subsoil, placing fresh, good soil in contact 

 with them, and giving liquid manure freely are the 

 best means to adopt for improving their condition. 



One form of clubbing in turnips and plants of the 

 cabbage tribe is caused by a fungus which attacks 

 ill-nourished plants, and the free use of superphos- 

 phate of lime is a good preventive. 



Another form is caused by the maggot of a weevil. 

 Small warts on the stems of young cabbage plants 

 should be cut off, and the roots dipped in a puddle of 

 earth, soot, and petroleum before planting. Lime 

 should be given unsparingly to the land, gas lime 

 also as advised for maggots. 



Under this category of enemies comes the terrible 

 potato disease. The fungus (Phytophthora) roots, so 

 to say, that is, spreads its mycelium through the tissues 

 of the potato leaf (A to B, page 79), and the stem 

 growths push through the breathing pores (c, E), for 

 plants, like animals, could not live if they did not 

 breathe. These stem growths of the fungus produce 

 " fruit "-spores (D,D) in cells (Oogonia) that divide 

 (F) and liberate the active agents in reproduction, 

 tailed zoospores (G), which float in the air, and 



