33 



hot, and dry weather, we are apt to have herbaceous plants in 

 such a condition as that described."* 



This would seem to explain why in some seasons the epi- 

 demic of wheat mildew is prevalent, and in some seasons absent, 

 and is confirmed in a most practical manner by the statements 

 of many of the agriculturists who answered the questions con- 

 tained in the Schedule, as to the coincidence of the attack with 

 the abnormal white frosts and the frequent and great changes of 

 temperature during the last summer. 



Moisture is necessary for the germination of the spores. It 

 has been considered that the moisture from the transpiration of 

 the plants is sufficient for their germination, at all events if it is 

 abnormal, or increased by unhealthy conditions. The entrance 

 of the germ-tube into the stomata of the wheat-leaf is easily 

 accomplished in normal circumstances, and is rendered easier if 

 the stomata are relaxed by influences unfavourable to the 

 health of the plant. Then, having gained an entrance, it would 

 appear to be dependent upon the nature of the food within its 

 reach in the cells of the wheat-plant, as to whether it forms 

 mycelium and continues its actively destructive work in trans- 

 porting the products of assimilation from the host to itself, as 

 Sachs puts it. "f" 



This is the usual action of what are known as " obligate " 

 parasites, or those which the host plant serves directly to sup- 

 port, and it is the action of the wheat mildew fungus, at least, 

 in its stages upon corn and grasses. In its other, or aecidium, 

 stage this action is modified, as the fungus appears then to lose 

 some of its obligate characteristics. 



This entire dependence of the fungus upon the wheat plant 

 naturally tends to injure it in proportion to the quantity of the 

 parasite upon it, if there is a continuance of conditions favourable 

 to the one and unfavourable to the host. The direct injury is 

 generally to the straw, which it blackens, disintegrates, and spoils 

 for every purpose but that of rough litter. 



Then in bad attacks the number of grains are lessened in the 

 ears. There are empty awns, and some of the grains are much 

 shrivelled, having lost weight, shape, quality, and colour. In less 

 severe attacks there is a percentage of grains more or less shri- 

 velled and distorted, which serves to spoil the appearance of the 

 sample. This is illustrated by Plate III., showing wheat from a 

 field infested by the fungus last season, though not very badly. 

 In this sample there were about 70 per cent, of grains plump and 

 well-shaped, as seen in Fig. A., and the rest more or less shri- 

 velled and deformed, as given in Fig. B. It will be understood 

 in this case, as in most cases of attacks of mildew, that there 

 was an unusual quantity of " tail " wheat, and it was necessary 

 to "run down" the wheat more frequently and closely than usual 



* Croonian Lecture, On some Relations between Host and Parasite in 

 certain Epidemic Diseases of Plants, by H. Marshall Ward, F.B.S. Pro- 

 ceedings of the Royal Society, 1890. 



f A Text book on Botany, by Julius Sachs. 



74690. C 



