966 
Journal of Agricultural Research 
Voi. xxra. No. 12 
qualitative and quantitative reduction of tissues, there exists, never¬ 
theless, a marked similarity between structure of normal host and wart, 
and this is shown not only in similarity of position of the tissues with 
reference to each other but also in their practical identity as regards the 
essential composition of the vascular tissue. 
MORPHOLOGICAL NATURE OF THE WART 
From anatomical analysis the'wart is a homoplastic growth with 
quantitative reduction in vascular tissue and increase in storage paren¬ 
chyma. The likeness is even more marked and reaches a point of com¬ 
plete identity if we choose to compare reduced host tissue with transi¬ 
tion regions in the wart. But even if we accept this conclusion, there 
still remains the answer to the question as to the nature of the wart in 
its entirety and its relation to the normal organs of the plant. To arrive 
at a satisfactory conclusion it will be necessary to follow briefly the early 
ontogeny of the wart and to consider the nature of the stimuli which 
initiate the abnormal growth. 
A newly infected bud shows the surface covered with small pustules 
which in sectional view (PI. 4, A) are found to consist of a small-celled tissue 
rich in protoplasm and very turgid. Certain of these protuberances 
show a slight depression with a brownish center in which is found a 
prosorus or summer sporangium of the parasite. These areas of new 
growth, according to Curtis (4) and Bally (j), have resulted from the 
stimulating effect of the fungus. Since the epidermal walls of these 
pustules are very delicate, and since new summer spores are formed and 
mature in rapid succession, new infections occur, and as a consequence 
new centers of growth are formed in direct proportion to the number of 
new infections. The finely adjusted stimulation which results in the 
formation of a progressively increasing area of meristematic tissue per¬ 
mits of an uninterrupted development of the parasite and the formation 
of large numbers of resting spores which under suitable conditions remain 
viable in the soil and form a latent source of infection for years to come. 
In semiresistant varieties wart development remains superficial. The 
growth slightly resembles scab pustules, though the infected areas are 
somewhat more elevated (PL 3, C, D). In susceptible varieties, on 
the other hand, the warts are extensive structures (PI. 2, A, D, C, E; 
3 A, B) and only in their topography bear resemblance to the former 
kind. Percival ( 8 ) assumes that the wart is “a malformed branch sys¬ 
tem stimulated by the parasite to grow irregularly and before its natural 
time.” A study of Plate 3, B, suggests no objection to such a theory, 
and a consideration of the anatomy of the vascular system seems only 
to lend additional support. However, granting that the stimulating 
effect of the fungus may result in a shortening or complete elimination 
of the rest period of the young bud, may we not expect a similar phe¬ 
nomenon in potato plants which are semiresistant to the wart? Yet this 
is not the case. Furthermore, an analogy with “ witches’-broom ” forma¬ 
tion in Pteris (5) indicates that it may be unessential, however, to pre¬ 
suppose the existence of buds in order to initiate growth of extensive 
though somewhat abnormal foliar organs; that, in fact, the stimulus 
exerted by the fungus is sufficient to bring about cell division and initiate 
organ formation in meristematic regions of plants. In the potato, too, 
extensive wart formation often results from the infection of an outer 
bud scale which has normally completed development. The association 
