418 Prof. H. Marshall Ward. The Relations between 



vapour in it, it must be borne in mind that many events concur in 

 promoting or retarding it. The stomata, for instance, open widely in 

 bright sunshine and close in the dark,* a matter of great importance 

 in controlling transpiration, as must be concluded from the researches 

 of Garreau and von Hohnel.f Other effects are traceable to the 

 influence of the wind shaking the plant, and to the quantities of 

 mineral salts, &c, in the soil, but it would carry me too far to discuss 

 further instances. 



The principal [effects of obstructed transpiration may be shortly 

 compared with those due to want of light — the watery tissues are 

 strikingly like those of an etiolated plant, and we may look upon a 

 shoot growing in a saturated atmosphere as presenting all the chief 

 features of one growing in darkness. J Its cells are extremely turgid, 

 with watery, soft, thin walls, and acid cell-sap ; its vascular bundles 

 feebly developed and hardly lignified ; and, as before, it is ill adapted 

 to withstand the exigencies of the ordinary environment. 



All such plants or organs are, so to speak, in a permanently young 

 condition. 



The Effect of the Preceding Variations in " Predisposing " the Host to 



Disease. 



If we put together the results of the preceding discussion, it is 

 evident that a plant may vary within very wide limits of the con- 

 dition we term health. No doubt this needs no proof to the minds of 

 most of my hearers, but the point I wish to emphasise is that, in some 

 of its deviations from, the normal, the plant offers conditions to an 

 attacking parasite which may be at one time favourable, at another 

 not. 



Suppose the case of a herbaceous plant growing under the follow- 

 ing circumstances in July : the temperature has been high, and the 

 daily supply of solar light abundant during the previous four or five 

 weeks, and everything has been going on admirably, so far. Suddenly 

 the weather changes — the temperature falls, rain sets in, and for many 

 days heavy clouds obscure the sun. If this markedly different, dull, 

 cold weather continues, we may have the following condition of 

 affairs more or less realised, as is well known to those who observe 

 cultivated plants closely. 



Transpiration being lowered in activity, the whole plant tends more 

 and more to be suffused with water; the stomata are nearly closed, 



* See Sachs, ' Lectures on the Physiology of Plants,' pp. 248—250, and Stras- 

 burger, ' Das Botanische Practicum,' 2nd ed., 1887, pp. 88 — 90. 

 t See Pfeffer, ' Pflanzen-Physiologie,' vol. 1, p. 144. 



X Yesque and Viet, " Influence du Milieu sur les Vegetaux " ('Ann. d. Sci. Nat.,' 

 G Ser. (Botanique), vol. 12, 1881, p. 167). 



