382 Ewart. — The Action of Cold and of 
a greater production of waste katabolic products, some of 
which (organic acids and their salts) are highly osmotically 
active. An increased turgidity is therefore to be expected 
in such cells, other factors remaining constant, and provided 
that the substances used up in katabolism are either replaced, 
or are not at all or only slightly osmotically active. It is 
perhaps in this way and by the softening of the cell-wall 
at certain points, that the fertilization-tubes are produced 
in Spirogyra . The subsequent contraction of the protoplasts 
before actual conjugation takes place might be either an 
active or a passive one, the water filtering out under marked 
pressure in the first case or slight in the second. If the 
contraction of the stretched protoplast is a passive one, then 
either the plasmatic membrane must have become more 
permeable, or else the osmotic concentration of the inclosed 
cell-sap must have undergone a rapid and marked diminution. 
That the latter is possible and even probable is shown by the 
fact that in the formation of the zygote a marked conversion 
of osmotic substances into non-osmotic ones takes place 
(sugars, &c., into oil, starch, &c.). In this way a rapid lowering 
of the internal osmotic pressure might be produced previously 
to conjugation. In any case the result is the same, a very 
resistant body, the zygote, being finally produced, which 
is very well adapted to withstand desiccation, extreme 
cold, &c. 
From all the facts mentioned above, it is easy to see 
that only by means of laboratory investigations, in which 
the experiments are performed under otherwise constant 
conditions, can any precise conclusions be arrived at with 
regard to complicated vital phenomena, nor is any de- 
termination of the resistant powers of a plant to any 
given agency possible, unless all other external agencies 
or modifying factors are allowed for, removed, or kept 
constant. 
That intense illumination acts injuriously upon all exposed 
living parts of plants is now quite certain, as may be seei\_ 
by reference to various sources, commencing with Pringsheim’s 
