92 
Mr. Webster thought that the matter needed more experiment ; that 
there was a prevalent opinion that mosquito larvae in ponds appropriated 
a good deal of organic matter that would otherwise become offensive, 
and by destroying them it was possible to do harm instead of good. 
The following paper was next read : 
PHYTOMYZA AFFINIS FALL., AS A CAUSE OF DECAY IN CLEMATIS. 
By Dr. J. Ritzema Bos, TVageningen (Xetherlands). 
[Read, in the author's absence, by H. Gasman.] 
For a few years a disease, formerly unknown, has been observed in 
various kinds of cultivated Clematis in the gardens of horticulturists 
in the Netherlands, especially at Boskoop. The affected plants have a 
diseased spot above the level of the ground; the lower parts are left in 
perfect health ; this can be said in particular of the roots. The parts of 
the stem lying higher than the diseased spot remain uninjured at first; 
they dry up, however, because they can not get a sufficient quantity of 
sap. On the affected spot all parts have become brown and have died ; 
in the first place the cells around certain very narrow mines, in the 
tissue of the stem. Especially the fasciculi vasorum have become brown 
in a high degree from the sick spot upward to some height. The sick 
spot is always recognizable on the outside. In the dead tissues I found, 
almost as a rule, a fungus of the genus Pleospora or a cognate one, and 
further a few kinds of Anguillulids. The parts of the stem above the 
affected spot dry up. Some systems of branches consequently die oft 
in a very short time, while others keep in good health. 
In one summer the sickness spread rapidly, so as to cause in a 
short time the decay of the superterrene parts of many plants, 
while under the affected spots new buds were shooting forth. The 
damage caused by this sickness was considerable. Above other 
varieties Clematis jacJcmani was strongly affected. At various times 
samples of sick Clematis stems were sent me, but I was not successful 
in my endeavors to make out the nature of the disease. In the "Zeit- 
schrift fur Pnanzenkrankheiten" of Prof. Sorauer I found mentioned a 
similar sickness in Clematis stems ; and the author of that treatise, Dr. 
H. Kletahn, at Bremen, gives as his opinion that the illness must be 
ascribed to the invasion of Anguillulids. He sent me sick Clematis 
stems for the purpose of a minuter examination of the Anguillulids ; but 
I found not one representative of the genera Tylenchus, Aphelenchus, or 
Heterodera, known to live generally as parasites in plants; all the 
Anguillulids I discovered belonged to genera without a spear, and 
these kinds are indeed sometimes found in decaying tissues, but com- 
monly do not live parasitically in plants. 
