19 



for photosynthesis being provided by respiration and the slow decay of the 

 organic matter in the humus. 



Insects and Plants. 



We are all familiar with the enormous losses produced every year by 

 the action of insects and fungous diseases on our cultivated crops. But 

 we should not fail to look at the other side of the shield, and ascertain how 

 many plants are dependent upon insects for their fertilisation and the pro- 

 duction of seed. In previous reports I have discussed this aspect in the 

 articles entitled "Adaptations between Plants and Insects (Third Report 

 pp. 43-45) and "Bees as Friends of the Agriculturist" (Fourth Report, pp. 

 71-95) and to those articles I beg to refer you for further details. To Dar- 

 win again are we specially indebted for his valuable researches in cross-and 

 self-fertilisation among plants. Let one example here suffice. He planted 

 14 seeds of cross-fertilised seeds of pansy and an equal number of self-fer- 

 tilized seeds. He found the average height of the plants arising from the 

 cross-fertilised seeds to be 5.58 inches, that from the self-fertilised seeds 

 2.37 inches. Next year the cross-fertilised plants produced 167 pods, the 

 self-fertilised plants produced 17 pods. In the year following the area 

 covered by the cross-fertilised plants was about nine times as large as that 

 covered by the self-fertilised plants. 



During the ensuing winter which was severe, all self-fertilised plants were 

 killed while the cross-fertilised plants survived and grew vigorously. Again, 

 seeds from cross-fertilised flow^ers or plants that came from self-fertilised 

 flowers, and seeds from cross-fertilised flowers or plants that came from cross- 

 fertilised flowers were planted. The average height of six plants fi*om the first 

 set was 10.31 inches, that from the second set 12.56 inches. 



With regard to red clover Darwin wrote:— "I have also found that the 

 visits of bees are necessary for the fertilisation of some kinds of clover — 

 — thus, 100 heads of red clover (Trifolium pratense) produced 27,000 seeds, 

 but the same number of protected heads produced not a single seed." 



Insects and Disease. 



In my paper in last year's report on " Insects and Disease" reference was 

 made to the important part played by insects in the spreacf of many diseases. 

 The Anopheles mosquito carries malarial organisms, the Stegomyia mosquito 

 carries the yellow-fever organism, the house-fly tuberculosis, typhoid fever 

 and other ailments, the stable-fly infantile paralysis, tsetse flies the terrible 



