60 DISPOSITION OF PLANTS TO DISEASE. 



their hosts are unprotected by cork-formation. Plant-organs 

 rich in water are in a condition which disposes them to attack, 

 much more than drier parts. The younger parts of any plant 

 are more disposed than older parts. Thus in a spruce-hedge 

 with young shoots appearing at different times, only those 

 shoots will be liable to attack, which are young at the time 

 of the scattering of the spores of Chrysomyxa dbietis, or other 

 spruce-fungus. De Bary was of opinion that plants of Gapsella 

 were disposed to attacks of Cystopus candidus, only as long as 

 they retained their cotyledons, because only those spores ger- 

 minating on the cotyledons form a mycelium which ultimately 

 finds its way through the plant, whereas plants which had 

 already lost their cotyledons at the time of infection were 

 in no danger. Many of the Ustilagineae attack cereals only 

 when these have just emerged from the soil, infecting the 

 young stems on the first sheath-leaf, whereas older and more 

 advanced individuals are exempt. While all plants with a 

 delicate epidermis or corky layer are liable to disease, yet some 

 are more so than others. This is exemplified by the different 

 powers of resistance to disease, or insect attacks exhibited by 

 nearly allied forms of our cultivated plants, e.g. vines ; a differ- 

 ence probably due to some variation in their outer membranes, 

 such as is further demonstrated by thick-skinned potatoes being 

 more resistant to disease than thin-skinned. 



Disposition is often due to external circumstances. These, 

 however, act rather in presenting favourable opportunities for 

 infection by germinating spores, than by directly disposing the 

 plant to disease. Thus prolonged wetting of a leaf from rain 

 favours germination of spores, and at the same time by softening 

 the leaf, facilitates penetration of the germ-tubes. Stahl ^ has 

 pointed out that leaves on which water remains for any length 

 of time, present greater opportunity for growth of saprophytic 

 epiphytes or for infection by parasites, than leaves with a 

 smooth surface or of a shape which facilitates ready escape of 

 water from their surface. It is also well known that larches 

 in damp situations suffer more from Peziza Willkommii than 

 those in drier places, the fungus-spores maturing and germinat- 

 ing only in moist air. Similarly, moist weather or damp 



' " Regenfall u. Blattgestalt," Ann. du Jardin hotan. de Buitenzorg, xi., 1893 

 p. 124. 



