6o 



favorable opportunity to germinate and develop themselves, whereas 

 with the Pythium the conditions of dissemination are much more 

 restricted. What is true in regard to the Botrytis is probably true in 

 regard to some of the mildews, as there is no reason to doubt that 

 the spores can thrive in the house for some time without coming in con- 

 tact with the host, although sterilizing the soil would undoubtedly kill 

 many of them. The so-called "drop" in the lettuce which is caused 

 by a facultative parasite, a species of Botrytis, is also completely 

 controlled by sterilization. This fungus causes no end of trouble to 

 some lettuce growers and is confined entirely to the soil where it 

 propagates only by means of its mycelium, but it frequently becomes 

 disseminated from one part of the house to the other by means of 

 the gardener's tools. Sterilizing the soil has also an effect upon 

 the weed and grass seeds which constitute more or less of a nuisance 

 in a house. The difference between a heated bed and one that is 

 not heated is very marked indeed in this respect. In the beds which 

 were heated at 204° F. there were no weeds or grass seeds to trouble 

 us and the only things appearing were one or two clover plants. 

 The seeds of the clover appear to be more resistant than other seeds 

 and their presence can be accounted for probably by the fact that 

 the temperature at certain points did not quite reach 204'^ F. In 

 the beds that were not heated we hoed under a number of crops of 

 weeds as the horse manure which was mixed with our soil was 

 largely contaminated with seeds. 



Relation of Nematodes to their Environment. 



A knowledge of the relationship of the environment to an organism 

 is of considerable importance in all experiment work where we have 

 to deal with some pest which causes injury to our economic plants. 

 Indeed some of the methods of controlling nematodes are based 

 upon a knowledge of the influence of the common external factors or 

 agencies which go to make up the environment and to which all 

 organisms strive to adapt themselves. Such for example is the 

 desiccation method which forms an important factor in the treatment 

 recommended by Vanha. 



The external factors playing an important part in the life history 

 of an organism are heat, light, moisture, etc., and it is the variation of 

 these ever changing factors with which the organism has to contend, 

 and which gives rise to characteristic manifestations in its activities. 



