370 A TEXTBOOK OF BOTANY [Ch. VII, 6 



of greenhouses or storehouses for fruits or root crops. Fungi 

 as a rule, in correspondence with their small size and para- 

 sitic mode of life within tissues, can stand bad air much bet- 

 ter than the higher plants, which are far larger, and physi- 

 ologically adjusted to more space and air. Accordingly, 

 bad ventilation alwaj's gives to Fungi an advantage over 

 their hosts, which explains why plants rot more freely in 

 unventilated than ventilated places. 



In our own times an attack has been made on the disease 

 problem along a very different line, viz. the effort to breed 

 immune varieties. The variability of plants is manifest in 

 susceptibility to a given disease as in other features, some 

 individuals of a kind being very susceptible and others much 

 less so. By a systematic selection of the latter individuals 

 it is found possible to breed immune races. Though the 

 problem is by no means so .simple in practice as in prin- 

 ciple, marked success has already attended the method, of 

 which we are sure to hear more in the future. 



6. Economics, and Cultivation, of Fruits 



The economic uses of fruits, apart from their seeds, which 

 will be considered later by themselves, center almost wholly 

 in the food value of the echble kinds, especially berries, 

 pomes, and gourds. It is notable, however, that fruits as 

 food are rather luxuries than necessities, having a relatively 

 slight food value though great palatability. This com- 

 bination, of course, comports perfectly with their function 

 of seed chssemination by agency of animals, where the 

 function does not require that animals shall be nourished, 

 but only attracted. Tlie dry kinds of fruits Ik^a'c hardly 

 any uses to man, the opium derived from young Poppy pods 

 being perhaps the most prominent economic protluct. 



As to the cultivation of fruits, the pliysiological and struc- 

 tural methods concerned have mostly been covered inci- 

 dentally in the earlier sections. Such are the pollination of 

 flowers by insects to insure fruit setting ; pruning to divert 



