668 
MOLECULAR FORCES IN THE PLANT. 
not unfrequently give rise to phenomena dilTering 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. (Nägeli), 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° C. as 1255 p. c, the starch originally containing, 
according to Nägeli, only from 40 to 70 p. c. water. 
Air-dry starch must be heated to nearly 200° 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 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 ß 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 ; y in c 
is the cavity of the cell ; e and r) are con- 
strictions at points where the outer layer is 
especially firm ; at S 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, lignified, nor 
converted into mucilage. 
^ See Sachs, Handbuch der Experimental-Physiologie, pp. 63 et seq. 
