. Text-book of botany, morphological and physiological. Botany. 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-walls1 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
. Text-book of botany, morphological and physiological. Botany. 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-walls1 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 500, or sometimes even 6o° 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 550 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 6o° C. as 142 p. c, at 700 to 720 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 2000 C. 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 500 to 6o° G., while
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectbotany, bookyear1882
