ON THE HEAT IN FORCING-HOUSES, 217 



attributable to the same cause. There are few peach -houses, or indeed 

 forcing-houses of any kind, in this country, in which the temperature does 

 not exceed, during the night, in the months of April and May, very 

 greatly that of the warmest valley in Jamaica in the hottest period of the 

 year : and there are probably as few forcing-houses in which the trees 

 are not more strongly stimulated by the close and damp air of the night, 

 than by the temperature of the dry air of the noon of the following day. 

 The practice which occasions this cannot be right : it is in direct oppo- 

 sition to nature : and I need not point out to the intelligent members of 

 the Horticultural Society, that the more nearly nature, in its best 

 climates and most favourable seasons, is copied as to temperature, the 

 more perfect will be the productions of the gardener's art. 



XXXI. ON THE MODE OF PROPAGATION OF THE LYCOPERDON 

 CANCELLATUM*. A SPECIES OF FUNGUS, WHICH DESTROYS THE 

 LEAVES AND BRANCHES OF THE PEAR-TREE. 



[Read before the HORTICULTURAL SOCIETY, December 5th, 1815.] 



I HAD the honour, two years ago, to address to the Horticultural 

 Society some observations upon the propagation of these supposed species 

 of parasitical plants, which, under the name of fungi-f*, appear as diseases 

 upon other living plants : and of other supposed species of the same tribe, 

 which decompose and feed upon organic substances, that have ceased to 

 live. In the present communication, I shall endeavour to show, that one 

 of these, at least, is a parasitical plant, which propagates like other plants, 

 by seeds. 



I observed, about seven years ago, a disease upon a few of the leaves 

 of one of the pear-trees in my garden at Downton. Bright yellow spots, 

 from which a small quantity of liquid exuded, appeared upon the upper 

 surfaces of the leaves in June; and subsequently, several conic processes, 

 about one third of an inch in length, were protruded from the same 

 parts, but from the opposite surface, of each leaf; and from these a 

 large quantity of brown impalpable powder, consisting of very minute 

 globular bodies, was discharged in August and September. These minute 



* I am indebted for the name of this species of fungus to the extensive information of Mr. 

 Dickson, who referred me to the Flora Danica for a delineation of it : but Sir Joseph Banks 

 subsequently showed me a drawing of it by Mr. Bauer, which is much more elaborate and 

 correct. 



t See page 204. 



