140 TRANSACTIONS OF THE ILLINOIS 



examination, mainly because less apples existed for its accommodation. 

 Some observations have been made, but not enough to spoil by being 

 kept over. At some future time a paper will be devoted to the subject, 

 if the Fates permit. 



Lettuce Mould. — T>ast winter Mr. Samuel Avery, of Champaign, 111., 

 undertook to grow lettuce for the Chicago and home markets in an ordi- 

 nary propagating house, heated by a common brick flue, with wood fire. 

 His plants did nicely until about half grown, when they began to decay, 

 the leaves becoming discolored, moist and rotten. He supposed the gas 

 escaped from the flue and caused the trouble, and took great pains to make 

 the brick-work tight, but the difficulty continued ; a hundred dollars, 

 worth had been destroyed, and more was on the same road. Without 

 stopping to name the other guesses and attempted remedies, I may state 

 the work was the result of a parasite belonging to the class of moulds 

 and known in the books as Peronospora gangUfonnis, Berk., closely allied 

 to the one causing potato-rot. In the case of the lettuce, the tips of the 

 tender leaves were first attacked, and the disease followed down the stem, 

 spreading and involving other leaves as fast as reached, until, finally, the 

 whole plant became a slushy, putrescent mass ; from one to two days 

 completed the destruction. The whole history of the germination of the 

 spore was not, perhaps, made out ; neither do I find it apparently full in 

 any works to which access is had. From its near relatives, we may, how- 

 ever, conclude that under some circumstances the spore {acrospore, or 

 conipium,) gives origin to from six to fifteen actively moving bodies, 

 known as zoospores, and these latter, after a play-time of from a quarter to 

 half an hour, cease moving, and germinate by emitting a slender tube 

 from their globular body. When in contact with a leaf these tubes pene- 

 trate the epidermis, and speedily spread themselves throughout the tissues 

 of the plant. At any rate this is sometimes the behavior of the spore 

 itself, a thing quite common among these plants of having more than one 

 way of accomplishin_g their purpose. Within forty-eight hours after the 

 germination of the spore, little white, tree-like, branching threads start in 

 tufts from the stomata of the leaves, and bear a second crop of spores, to 

 begin work again upon other plants, having been disseminated by the air 

 currents. Later, inside the decaying tissues of the plant, another form of 

 spore (oospore ) is produced. This originates from a kind of sexual union 

 of two portions of the vegetative threads (mycelium) of the fungus. On 

 one thread a globular vessel (oogonium) is formed, holding a rounded 

 protoplasmic mass. On another thread a smaller swelling appears, and 

 its contents become separated by a partition from that of the rest of the 

 thread ; the latter protuberance (antheriddium) is applied to the former, 

 and soon emits a tube, which perforates the wall and reaches the globular 

 mass inside. This latter now becomes the oospore, and is much more 

 capable of withstanding the vicissitudes of prolonged existence than other 

 spores. Ordinarily this <?(?j/(9r^ bridges the winter and starts growth again 

 in the spring, all other parts of the plant perishing. But, as if not to be 

 beaten under any circumstances, still another method of preservation is 

 sometimes resorted to. Having a diseased leaf under a bell-glass in a 



