98 PLANT GROWTH 



a plant, it is important to keep all of the leaves covered with 

 a protective spray and so kill all of the organisms before they 

 enter the plant. It is usually best to destroy a diseased plant. 

 Although we have very little evidence to indicate that a 

 "sick plant can be treated as we treat an infected animal" 

 recent work on the control of the red spider in greenhouses 

 suggests a possible approach to the problem. Since plants 

 containing the poisonous selenium were found to repel red 

 spider very small quantities of selenium as sodium selenate 

 are now being successfully applied as a fertilizer for green- 

 house plants. 



There are several aspects of this problem encouraging to 

 the gardener. (1) The epidermis of the leaves protects the 

 plants against attacks of insects and diseases. (2) Many 

 sprays are available which, if applied properly, will give 

 almost complete protection. (3) Many of the sprays stimu- 

 late growth sufficiently to pay for the cost of spraying by 

 increased yields even though no disease control was neces- 

 sary. (4) Spray combinations may be used to give control 

 of all the insects and diseases in the same solution. (5) 

 Plant breeders have been able to produce varieties or strains 

 which are less susceptible, as described in the chapters on 

 plant breeding. 



The unpalatable, tough epidermis with its cutin or hairy 

 covering protects many leaves, as may be seen by comparing 

 such leaves with those which insects select in a garden. In 

 many cases the time of the opening of the stomata may be 

 important in restricting the entrance of the germ tube from 

 a spore, because if a germ is in dry air for a short time during 

 its development before entering a stoma it is killed. In other 

 cases the fungus can penetrate the epidermis with difficulty. 



Most of the diseases are spread by spores, carried by the 

 wind or by insects. They are so numerous in regions where 

 plants have been growing that it is impossible to grow plants 

 on which spores do not fall. When moist, spores begin to 



