402 NATURAL SCIENCE. June. 



cases in which the leaves fall, showing either a complete brown or 

 yellow discolouration, or only a partial one limited to the apex, margins, 

 or base, or restricted to minute specks, or irregular patches scattered 

 over the surface. Sometimes the leaf falls uninjured ; according to 

 Mr. Watson, during some recent fogs bushels of healthy-looking 

 leaves were gathered up almost every morning in the Palm House at 

 Kew. 



The local blotchings of the first class are presumably due to the 

 action of an acid on the upper surface of the leaf. The layer of dirt 

 deposited by the fog contains an appreciable amount of sulphuric acid, 

 the product of oxidation of sulphurous acid. The frequent wetting of 

 the leaves brings this into solution and the drops having a tendency 

 to collect at the tips and margins will leave there on evaporation a 

 deposit of acid. This process is continually repeated until sufficient 

 acid accumulates to corrode the surface. 



The changes in the leaf which cause its rapid fall, with or with- 

 out a colour change, are, the author thinks, largely due to an attack 

 on its delicate unprotected internal tissues. The fog effects an entry 

 through the stomates into the system of intercellular sf)aces, where 

 its poisonous ingredients come in immediate contact with the moist, 

 delicate, and unprotected membranes of the living cells, which offer 

 but little resistance, and the protoplasm is directly attacked. 

 Whether the leaf succumbs or not depends on the inherent consti- 

 tution of the protoplasm. The process was carefully followed in leaves 

 of Rhododendrons and others, and the action was found to begin in 

 the lower layers of the spongy tissue next the stomates. Thence it 

 spread to the upper parts of the leaf and the epidermis. In the case 

 of thin uncuticularised leaves when the epidermis is very soft, the 

 noxious vapours may enter directly through the outer layer, as well as 

 by the intercellular spaces. It is not clear to what component of the 

 fog this second class of injuries is due. Owing to the very general 

 absence in the injured leaves of acid products of the green colouring 

 matter, sulphurous acid cannot be the sole agent, and Professor 

 Oliver thinks that some complex organic bases like pyridine may 

 play an important part. As regards remedial measures, there seems 

 but little to be done, except, as Lindley long ago suggested, to keep 

 the temperature as low as is compatible with the life of the plant, 

 with sufficient humidity to avoid desiccation. If the houses are 

 otherwise tolerably air-tight, a contrivance for filtermg the incoming air 

 through boxes containing trays of charcoal sticks may be used to 

 mitigate the evil. 



A Technical Education Conference. 



On April 20 and 21 the Senate House at Cambridge was the 

 scene of a Conference on the relation of Universities to the County 

 Councils in regard to technical education. 



