a Lecture hy Trofe^^or Lindley. 331: 



ever they touch. They destroy a plant by covering the sur- 

 face of its leaves, and thus preventhig respiration. Plants 

 are generally most affected by superficial fungi after a long 

 drought, when the fibres of their roots are unable to imbibe 

 sufficient moisture from the soil, and the plant thus becomes 

 debilitated, and affords an easy prey to the parasite which 

 attacks it : as a proof, Dr. Lindley mentioned that in Scot- 

 land, where there are heavy night dews, this fungus is un- 

 known. The cure seems to be abundant watering. Dr. 

 Lindley mentioned a case of some onions, in the gardens of 

 the Society at Chiswick, which were attacked by this fungus. 

 These onions had been transplanted, and their roots were 

 consequently so much weakened as to be unable to imbibe a 

 sufficient quantity of moisture. Dr. Lindley had the plants 

 abundantly watered, which, though it did not cure the in- 

 fected plants, appeared completely to arrest the progress 

 of the disease. Other onions, not transplanted, were not 

 attacked. A correspondent of this Magazine (Mr. Hay- 

 croft, Vol. VIII. p. 40.) recommends a mode of curing this 

 mildew, which appears to be effectual, by cutting off the 

 infected branches, washing the walls with a composition, and 

 removing the infected nails, &c. Sulphur has also been 

 recommended, but is not found to answe]'. [For other 

 notices, in this Magazine, on the mildew, see Vol. III. p. 1 19., 

 Vol. IV. p. 281. 320., Vol. V. p. 107., Vol. VII. p. 87.] 



The internal mildew evidently cannot be communicated by 

 the air, since it always appears to spring from the interior of 

 the plant, and to be at first covered witli a thin skin, from 

 which it does not burst till it is ripe. It is impossible, there- 

 fore, that this kind of mildew can be communicated exter- 

 nally, and yet the fact that it is contagious is so clear as not 

 to admit a doubt. The only manner in which it appears 

 probable that it can reach the interior is through the roots. 

 The seeds, when ripe, fall upon the earth, which becomes 

 contaminated by them, and they are sucked up by the 

 spongioles of the roots. [Mr. Dovaston has, also, held this 

 opinion : see Vol. VIII. p. 739.] The correctness of this 

 hypothesis is proved by sowing clean seeds in infected soil ; 

 and the young plants from these seeds springing up with the 

 disease upon them. The circumstance of its always attacking 

 the most vigorous plants is thus also explained, as it is 

 evident that, the more rapid the circulation, the greater is 

 the probability of extraneous substances being drawn up with 

 the moisture imbibed by the roots. It is also clear that, in 

 this case, water must aggravate the disease; as, by exciting the 

 plant to suck it up, it would increase the danger of the seeds 

 of the fungi beinff drawn in with it. This was also the 



