SECTION OF VEGETABLE PATHOLOGY. 365 
they occur indifferently on any part of the leaf, often running to- 
gether, forming large brown patches, and m severe cases they occupy 
the greater part of its surface, seriously interfering with the process 
of assimilation. They are caused My the growth of the parasite 
within the tissues, whose limited development causes the sharply- 
defined, rounded outline of the spots. ,The tissues are destroyed 
through the entire thickness of the leaf, and this portion often breaks 
away completely from the surrounding healthy parts, leaving the 
leaf full of round holes. Fungi producing this effect are sometimes 
named ‘‘shot-hole fungi.” In advanced stages of the disease the 
leaves become wrinkled and deformed; finally, losing all vitality, fail 
to the ground. 
(c) MICROSCOPICAL CHARACTERS, 
A section through one of the spots where it joins the healthy tis- 
sue shows a well-defined boundary in the tissues by a peculiar trans- 
formation of the cells. In the yellowish-green border by which 
most of the spots are surrounded the different leaf tissues become a 
homogeneous mass of parenchyma; within this the tissues are so 
shrunken and altered that the cells have lost all definiteness of form 
and contents, forming a brown and disorganized mass (Fig. 1, b). 
This disease appeared to be sufficiently important to demand fur- 
ther investigation, and some time was occupied in this work. 
Several different fungi were found on the dead spots, those worth 
of note being Macrosporium catalpe and Phyllosticta catalpe. Both 
these species are described by mycologists as forming spots; both 
were observed on the same spot, and the mycelium of both was 
found penetrating the tissues. 
The Macrosporvum is the more abundant of the two, being gen- 
erally constant on both sides of the leaf, excepting on the very 
young spots. Its growth sometimes extends beyond the limits of 
the latter to the green tissues immediately surrounding them. The 
conidiophores, or spore-bearing stalks of this fungus, are reddish- 
brown; they occur singly or in groups, and on the under side of the 
leaf they were frequently seen emerging from the breathing pores or 
stomata (Fig. 2). The spores (Fig, 2, b, and 3) are compound or 
made up of a number of cells, each cell or division being capable of 
ready germination (Fig. 3). The mycelium produced by germina- 
tion is comparatively large. colorless, frequently divided by septa, 
and its growth is readily carried on in a variety of media. 
The sowing of fresh spores on healthy leat surfaces, an experi- 
ment frequently tried, led to no positive results. 
The species of Macrosporium are so frequent on dead vegetable 
substances and so rarely, if ever, found alone on living tissues that 
they are generally believed purely saprophytic in their habits and 
incapable of producing direct injury to living plants. If this be 
universally true, then we must look to the other fungus mentioned 
as the cause of the leaf-spot disease under discussion. 
The Phyllosticta, many species of which are known to produce 
leaf-spot diseases in other plants, found on the spots on the Catalpa 
leaves is doubtless the cause of them. Its mycelium is very deli- 
cate, being much finer than that of the Macrosporium, so that it is 
very difficult to see it in diseased substance of the spots, even by the 
most careful manipulation. The spores of this fungus (Fig. 5) are 
formed within minute black conceptacles or pycnidia (Fig. 4), that 
are developed just beneath the cuticle of the leaf which their growth 
