108 BULLETIN OF THE BUSSEY INSTITUTION. 
does not recur every year, but that there are intervals of inactivity ; or, 
if we suppose that the swelling is due to the fact that certain insects 
sting the buds, which then take on a monstrous development, we must 
suppose that the insects do not make their appearance regularly every 
year. 
Of the various excrescences on trees, classed popularly under the 
name of knots, it is certainly true that but comparatively few of them 
are the results of fungus growth. Many are known to result from 
the irritation caused by the stings or punctures of insects; but still 
there is a very large number of cases about which nothing definite is 
known. Of this large number of doubtful cases, it is by no means 
unlikely that a good share will prove on further observation to origi- 
nate in the attacks of insects. Were they of fungoid origin, it is 
almost impossible to suppose that at least traces of the fungus causing 
the trouble should not be found in the knot, even when tolerably old ; 
whereas it is well known that some of the largest knots arise from 
punctures of buds made by very minute dipterous insects, which are to 
be found only during a short time, and even then with difficulty. The 
whole subject of the minute anatomy and growth of swellings in plants 
caused by insects is of great importance, and as yet almost entirely 
unexplored, partly from the reason that it implies on the part of the 
investigator a very considerable knowledge of entomology and vegeta- 
ble anatomy as well as an acquaintance with fungi. It may be asked 
whether some of the excrescences of plants do not arise from some 
spontaneous morbid action of the cells of the plant just as is the case 
with many tumors in the human body. ‘That some such spontaneous 
morbid action occurs, is certainly true; but we have very little definite 
knowledge of the matter, and the experience of vegetable pathologists 
tends towards the belief that morbid growths in plants more frequently 
arise from the presence of fungi or insects than from spontaneous 
changes in the cells themselves. 
No. 22 of the Bulletin was an article on what, from its frequent 
occurrence in this country, we called the American grape-vine mildew, 
Peronospora viticola. 'The frequency of the disease has been ques- 
tioned by some, but there can be no doubt that it is very common 
indeed in New England, both on wild and cultivated grapes, and, inas- 
much as specimens of grape leaves received from several of the 
Western States are also covered plentifully with the fungus, in all 
