206 Linnaan Society. 



Mr. Stark sent specimens for exhibition of the following Algae 

 dredged in the neighbourhood of Lamlash, Arran : — Bonnemaisonia 

 asparagoides, Halymenia ligulata, Polysiphonia parasitica, Delesseria 

 ruscifolia and Nicophyllum punctatum. 



LINN^AN SOCIETY. 



November 7, 1848. — Edward Forster, Esq., V.P., in the Chair. 



A paper was read by F. J. Graham, Esq., F.L.S., " On the In- 

 juries sustained by certain Plants from the attacks of parasitic Fungi, 

 with particular reference to the Cause of the Potato Disease." 



In order to demonstrate the subject more clearly, Mr. Graham 

 exhibited drawings, with magnified figures of several species of para- 

 sites ; and a great many specimens of different plants, both native 

 and exotic, presenting a healthy appearance on those parts which 

 were still free from the attacks of the different species of mildew to 

 which they were subject, but at the same time showing the most in- 

 disputable signs of disease on those parts which were infested by 

 tufts of mildew. The manner in which one plant in particular. 

 Shepherd's Purse {Thlaspi Bursa Pastoris, L.), was affected, was very 

 remarkable. Portions of the stems of this were covered, to the ex- 

 tent of two or three inches, with Botrytis parasitica, which caused 

 them to become gouty or swollen to three times their natural size ; 

 and eventually these parts assumed a brown colour and a moist pu- 

 trescent character, which could be traced down the stalks, and in 

 many cases killed the plants. Transverse sections of these blotches, 

 compared with similar sections of a blotch on the potato stalk, ex- 

 hibited the same effects, the dark fluid having penetrated the tissues 

 of both to a considerable extent. Of all the species of parasitic mil- 

 dews which he has noticed, Mr. Graham considers those belonging 

 to the genus Botrytis to produce the severest injuries ; and it is an 

 undisputed fact that the potato crops have been universally attacked, 

 during the last three seasons, by Botrytis infestans. 



As to the manner in which these parasites acquire their destruc- 

 tive power, Mr. Graham considers that it arises from the natural de- 

 cay of their mycelium or internal filaments, which he has found tra- 

 versing the tissues of plants, beneath the external tufts of mildew. 

 That the tissues of plants are extensively permeated by this myce- 

 lium, has been frequently shown by the Rev. M.J. Berkeley and other 

 mycologists ; but the important fact that these roots (as they may 

 be termed) die within the tissues of plants, along with their super- 

 structure, assuming a dark colour in decay and ultimately dissolving 

 into a viscous mass, has hitherto, Mr. Graham states, escaped the 

 notice of authors. Decaying matter being thus secretly introduced, 

 corrupts the adjacent tissues, and in many cases spreads over the 

 entire plant and kills it. Mr. Graham states that he has arrived at 

 this conclusion after repeated examinations under powerful micro- 

 scopes, but that the effects are visible in some cases to the naked 

 eye. Experiments made by enclosing tufts of mildew in the sap of 

 those plants on which it grew, also exhibited the results above stated. 



