668 



MOLECULAR FORCES IN THE PLANT. 



not unfrequently give rise to phenomena differing not only in degree but even in kind. 

 The effect of most external influences depends moreover to a great extent on the 

 chemical nature of the substance which forms the material and micellar framework of 

 an organised body. Cell-walls ^ and starch-grains for instance differ from crystalloids, 

 chlorophyll-granules, and protoplasm, since the former consist mainly of carbo-hydrates 

 insoluble in water, the latter chiefly of albuminoids. 



The following are some of the more obvious phenomena selected from the great 

 mass of existing observations, which are, however, still incomplete. 



(a) Temperature does not usually cause any striking 

 or permanent change or destruction of organisation till 

 it exceeds 50°, or sometimes even 60° C, even when 

 the substance affected is completely saturated with 

 water. Air-dry organised bodies can generally bear 

 much higher temperatures without injury. Thus, for 

 example, the denser portions of a starch-grain which 

 is saturated with water are not converted into paste 

 below 65° C, while the more watery portions undergo 

 this change at 55° G. (Nageli), the capacity for absorb- 

 ing water and in consequence the volume then in- 

 creasing enormously. Payen gives the increase in 

 volume of starch in water at 60° G. as 142 p. c, at 70° 

 to 72° G. as 1255 p. c, the starch originally containing, 

 according to Nageli, only from 40 to 70 p. c. water. 

 Air-dry starch must be heated to nearly 200° G. before 

 its power of absorbing water materially increases ; but 

 it is then changed chemically and converted into dex- 

 trine. The corresponding action of temperature on 

 cellulose is not yet accurately known, but it is certainly 

 different from that on starch. Like albuminoids, proto- 

 plasmic structures consisting for the most part of these 

 substances are, when saturated, coagulated by a tem- 

 perature of from 50° to 60° G,, while when air-dry they 

 can stand much higher temperatures without their 

 micellar structure being destroyed ^. The remarkable 

 difference in the action of temperature on saturated 

 starch on the one hand and on saturated protoplasm 

 on the other hand must not be overlooked. In the 

 former case the power of absorbing water is enor- 

 mously increased ; its structure becomes looser and 

 more easily susceptible to chemical action ; while the 

 coagulation of protoplasm diminishes its power of ab- 

 sorbing water and the mobility of its micellae, and 

 increases its power of resisting chemical action. This 

 difference is also manifest when the change of micellar 

 structure is caused by acids ; and in this respect normal 

 cellulose behaves in a similar manner to starch. 

 (b) Acids (especially sulphuric acid) when greatly diluted cause starch-grains and 

 cellulose at the ordinary temperature to swell up much more violently than pure water, 

 without however destroying their organisation ; and the previous condition returns when 

 the acid is washed out. If, on the other hand, the acids are more highly concentrated, 



FIG. 466.— Trichoblasts from a leaf of 

 Hoya carnosa (see Fig. 30, p. 29) ; a and b 

 after the commencement of the action of 

 iodine and dilute sulphuric acid; c, when 

 the swelling in dilute sulphuric aeid has 

 proceeded further, a. and /3 in a are the 

 outermost layer not capable of swelling, 

 and coloured dark-blue, which breaks up 

 somewhat irregularly in these cases, but in 

 c more regularly, into a spiral band, while 

 the inner layers swell between them, and 

 are coloured light-blue by iodine ; 7 in c 

 is the cavity of the cell ; e and >) are con- 

 strictions at points where the outer layer is 

 especially firm ; at 5 the greatly swollen 

 substance is beginning to become disor- 

 ganised (X 800). 



^ The cell-wall I suppose here and in the sequel to be neither cuticularised, lignifiied, nor 

 converted into mucilage. 



See Sachs, Handbuch der Experimental-Physiologic, pp. 63 et seq. 



