410 Prof. H. Marshall Ward. The Relations between 



plant. These factors are such as the following : changes of tempera- 

 ture, variations in the intensity of the light, differences in the amount 

 of aqueous vaponr in the atmosphere, &c, in short, the variable 

 factors of the physical environment of the plant. That these affect 

 the physiological processes in the cells is well known ;* but what I 

 "have to do is to trace some of the effects, and show how they bring 

 the living tissues into such conditions that they more or less readily 

 resist or succumb to the attacks of certain parasitic fungi. 



Taking the more or less arbitrarily chosen but convenient headings 

 already employed — respiration, metabolism, growth, carbon assimila- 

 tion, &c. — let us now see what kinds of effects the external agents 

 referred to may produce. 



Respiration, though it proceeds at very low temper atures,f is ren- 

 dered considerably more energetic as the temperature rises, until, 

 after a certain relatively high temperature (about 45° C.) is reached, 

 it becomes less intense, and injury to the cells soon results, un- 

 doubtedly from damage to the structure of the living substance, 

 owing to the excessive disturbances brought about in its metabolism. 

 Speaking generally, we may fairly say that at temperatures near 0° to 

 5° C. the respiration is very slow ; as the temperature rises the respira- 

 tory activity increases, at first slowly, and gradually more and more 

 rapidly, till at 35° to 45° C. it is at its maximum intensity ; beyond 

 that it rapidly declines, and ceases with the death of the protoplasm 

 at about 50° 0. 



Light appears to exert little or no effect on the normal process of 

 respiration, unless relatively very intense,^ when it may possibly 

 promote it; but bright light may accelerate certain processes of 

 oxidation which would otherwise have gone on more slowly.§ This 

 much probably may be said, however : in so far as light influences 

 oxidation processes (other than respiration) in the living cell, the 

 action increases with the intensity of the light. || As we shall see 



* See Sachs, ' Lectures on the Physiology of Plants/ pp. 189—204, 299—308, 

 552 — 555, &c, for an introductory general account. The special literature Trill be 

 noticed as we proceed. 



f See Kreussler in ' Landwirthschaf tiiche Jahrbucher,' vol. 16, 1887, and vol. 17 r 

 1888, for the dependence of respiration on temperature. 



X Of course referring to ordinary daylight only. 



§ See Pfeffer, ' Pflanzenphysiologie,' vol. 1, p. 376, as to possible bearing of this 

 on decomposition of organic acids (Pfeffer, " Uber d. Oxydationsvorgange in leben. 

 Zellen," pp. 454, 469, 472). 



|| As to the effect of very intense light, see Pringsheim, " Ueber Lichtwirkung 

 und Chlorophyllfunction in der Planze " ('Jahrb. f. wiss. Bot.,' vol. 12, 1880, 

 pp. 84 — 93). It should be remarked that Pringsheim not only shows that the 

 action is really due to light- and not heat-rays, but that the more refrangible rays 

 (blue, &c.) are the most active (see pp. 40 and 52). These and others of Pringsheim's 

 observations may be accepted without prejudice as to his theory of assimilation. 



