198 



MONOGRAPH OF THE LABOULBENIACE^E. 



little if any appreciable injury on the host, and even when the latter is completely 

 covered by them it shows no more marked signs of injury than is indicated by a 

 greater restlessness, owing perhaps to a slight irritation which they may be supposed 

 to produce. This absence of appreciable injury, associated as it is with true parasitism, 

 is due to the fact that the habit of growth of the plants in question is an external one, 

 unassociated, except in rare instances, with any penetration of well-developed haus- 

 toria into the body cavity, the parasite in almost all cases deriving its nourishment 

 through at most a slight perforation of the host's integument. The hosts affected 

 are all comparatively long-lived hibernating insects, and more or less continuous 

 feeders; and in the present, as in so many other instances, are obliged to become 

 the unwilling medium for the nutrition of an often numerous and varied population 

 from which they are freed only by death. 



An external parasitism, like that of the plants in question, on hosts living and as a 

 rule actively locomotive, whether in water, in the air, or on the ground, would natur- 

 ally be associated with a comparatively simple structure adapted to the exigencies of 

 such a life ; and a glance at the accompanying plates will show that such a simple 

 type form may be traced in a general way throughout the group. A main body, or 

 receptacle, is fixed by means of a blackened base, or foot, to the integument of the 

 host, and consists in most cases of a very small number of cells differently arranged in 

 different genera. This receptacle gives rise above to certain peculiar appendages of 

 very variable form, commonly connected with the production of the male sexual 

 organs; while from the same individual, with few exceptions in which the plants are 

 dioecious, female organs are also variously produced from which perithecia are event- 

 ually developed. In the perithecia, which may arise singly or in considerable num- 

 bers from a given individual, and which are quite remarkable in structure, are 

 produced the reproductive bodies or ascospores that 7 are formed in asci identical in all 

 respects with the organs thus named in other members of the great group of aseomy- 

 cetous fungi. The ascospores thus formed germinate on the surface of the host to 

 which they become attached by a blackened modification of their basal extremity, 

 and, without the formation of any hyphce, grow directly to new individuals by means 

 of successive cell divisions. In respect to size the mature individuals vary consider- 

 ably within certain rather narrow limits, for while the smallest species measure some- 

 what less than one-tenth of a millimeter in total length, a very few exceed a millimeter 

 from base to tip, while by far the greater number do not attain more than half this 

 length. Within the limits of size and fundamental structure just described, the op- 

 portunities for eccentricity of form seem, however, to have been abundantly utilized, 



