190 



XXXIIL— HOW SAPROPHYTIC FUNGI MAY 



BECOME PARASITES. 



G. Massee. 



It is very generally believed that diseases of plants, caused by 

 fungi, continue to increase in number, and in the case of cultivated 

 plants this is probably true, not because the adaptive power of 

 fungi is greater than at any jjrevious period, but simply because 

 the opportunities for exercising the adaptive power possessed by 

 fungi are more frequent at the present time, too often due to 

 a departure from the normal on the part of the host-plant, brought 

 about by cultivation. 



Quite recently my attention was called to a batch of 

 Clerodendron fallax^ LindL, in one of the houses at Kew; the 

 gardener had noticed the presence of numerous minute watery- 

 looking drops^ on the under surface of the leaves, an unusual 

 phenomenon, not to be found on another batch of the same kind 

 of plant growing under slightly different conditions as to tempera- 

 ture and moisture. On investigating the matter it was found that 

 the under surface of the leaf, more especially towards the base, 

 was studded with comparatively large, peltate glands, supported 

 by a very short central stalk. These glands each exuded a liquid 

 drop which had a very sweet taste. The ubiquitous floating spores 

 of Cladosporium epiphyllumj Pers,, found these sugary drops a 

 congenial pabulum, and each gland was soon tipped with a fruiting 

 tuft of Cladospormm. At first the Cladosporium was strictly 

 confined to the glands, and depended on the secretion for its 

 support, but it gradually passed from the saprophytic condition, 

 and entered that of a facultative parasite, passing beyond the 

 range of the gland and attacking the surrounding living tissue of 

 the leaf, forming conspicuous brown, dead patches on the upper 

 surface. Three weeks after the disease first appeared, the spores 

 of the fungus were capable of infecting any portion of the leaf, 

 quite apart from receiving an initial start on the sugary excretion 

 from a gland. The above is a concrete example of a saprophytic 

 fungus becoming a parasite within a brief period of time. It is 

 unlikely that all the conditions necessary to effect this change will 

 ever occur again, hence the epidemic will be of short duration, but 

 it can be readily imagined that if the host-plant had been an out- 

 door crop, and the epidemic had remained unchecked, the fungus 

 might have become a pronounced parasite, capable of continuing 

 its ravages for all time. 



Judging from the number of examples sent to Kew for deter- 

 mination, mechanical injuries of various kinds, often self-inflicted, 

 are a source of perplexity as to their origin. Wind is the most 

 important factor. The gourds that are trained up poles in the 

 herbaceous ground furnish striking examples. When a young 



fruit happens to be overhung by a leaf, and the latter is gently 

 swayed to and fro by the wind, the rigid hairs on the under surface 

 of the leaf form a series of more or less parallel scratches on the 

 surface of the fruit. As the direction of the wind changes, the 

 series of lines on the fruit cross each other diagonally, or form a 



