Notices oe books and memoirs. 677 



was distant at the portion to which I refer at least live yards from 

 the nearest potato plant. During the five days and nights of the 

 experiment a gentle wind blew, and the weather was, on the whole, 

 dry and clear. Every morning about nine o'clock I placed fourteen 

 slides on the lee side of the field, and every evening about seven 

 o'clock I removed them, and placed others till the following 

 morning at nine o'clock. Each slide was carefully examined with 

 the microscope immediately before being placed in its position, 

 and I took the precaution of spending the greater part of each day 

 in the neighbourhood to prevent disturbance or communication 

 between the potato field and the slides by any discernible agency 

 except the atmosphere. So far as it was conceivable and possible 

 to me, the slides were isolated during the period of exposure from 

 every agency but the atmosphere. After the slides were placed in 

 position they were never approached until I did so to replace them 

 by others, and this I was careful to effect from the side of the wall 

 more remote from the potato field. The examination of them 

 began within thirty minutes after their removal from the wall. 

 The field was not watched during the night ; but as no conidia fell 

 on the slides during that period, the absence of this precaution 

 cannot invalidate any result. On no occasion, however, did I 

 discover any sign of the slides having been disturbed. The four- 

 teen slides exposed during the day, when examined in the evening, 

 showed (among other objects) on the first day, 15 conidia ; on the 

 second, 17 conidia; on the third, 27 conidia; on the fourth, only 

 4 conidia ; and on the fifth, 9 conidia. On none of the five nights 

 did a single conidium alight on the slides. This seemed to me to 

 prove that during the day the conidia, through the dryness of the 

 atmosphere and the shaking of the leaves, became detached and 

 wafted by the wind; while during the night the moisture (in the 

 form of dew, and on one occasion of a slight and gently falling 

 shower) prevented the drying of the conidia, and thus rendered 



them less easv of detachment. 



« I determined the authenticity of the conidia (1) by com] ring 

 them with conidia directly removed from diseased plants; i-) by 

 there being attached to some of them portions of the characteristic 

 coindiophores ; and (3) by cultivating them in a moist chamber 

 the result of which was that five conidia, not having been immersed 

 fthe glvcerine, retained vitality, which was shown by then 

 Tursting and producing zoospores in the manner characteristic of 



these organs ^ ^ ^.^ ^ ed 72 i(U 



fell during 50 hours from a field bearing about two per cent of 

 diseased plants shows, in my opinion, that the diffusion of the 

 conTdla by the atmosphere is an ordinary mode of spreading the 



^ There were perceptibly more diseased plants in the field on 

 the fifth day than on the first, but fewer than might have ben 

 exnected from the number of conidia presumably in the atmo- 

 sphere For about a week after these experiments I cursorily 

 examined the field each day, and on the fourth day a sudden m- 



