k j 
MR. A. J. EWART ON ASSIMILATORY INHIBITION. 401 
coincident. All plants which can withstand desiccation can also 
withstand freezing ; but it is only plants which are very intolerant 
of, and very readily damaged by, a loss of water that are easily 
killed by freezing at a few degrees below zero. “o very low 
temperatures, since the withdrawal of water is here complete, 
only those plants are resistant which are capable of withstanding 
complete desiccation *. 
The latent period of return of assimilation after exposure to 
cold is nearly always of relatively short duration, and is possibly 
produced by the withdrawal of water from the protoplasm, and 
represents the time necessary for the assimilatory plasma to 
completely return to its normal condition of metabolic activity. 
As regards the temperature at which assimilation commences, 
Cloez and Gratiolett found that it began in Potamogeton Cerato- 
phyllum and Conferva between 10° C. and 15° C., and in Vallisneria 
above 6°C. With larch-leaves Boussingaultzt detected an evolution 
of oxygen at 09:5 C. to 2°5 C., and with meadow grasses at 1^5 C. 
to 3*5 C.; whilst Heinrich$ found that with Hottonia palustris 
bubbles of oxygen were first given off at 2^7 C. By the more 
delicate Bacterium method an evolution of oxygen can be detected 
at temperatures several degrees lower than the above. It appears 
that all evolution of oxygen ceases in tropical plants between 
4? C. and 8 C., in warm temperate, subtropical, and water-plants 
between 0? C. and 2° ©., whilst in cool temperate, arctic, and 
alpine plants assimilation only ceases when the plauts are frozen, 
i. e. at a few degrees below 0^ C. In this last case the stoppage 
of assimilation is of purely physical origin, and is due to tue 
withdrawal of water from the protoplasm to form ice erystais, 
leaving the assimilatory plasma in what is virtually a more or less 
completely desiccated condition. 
In many cases freezing and the stoppage of assimilation are 
coincident with a cessation of vitality, but in others the plants 
may be completely trozeu and assimilation stopped without the 
power of recovery being lost. In such plants both respiration 
and assimilation may function at zero, or eveu, and especially 
the former, a few degrees below it. Thus Clausen finds that 
* See Pfeffer, ‘Physiologie,’ Bd. ii. Cap. 10. 
t Cloez and Gratiolet, in Journ, univ. Sc. France et à l'étranger, 30 Oct., 
1850, No. 878. 
I Boussingault, in Ann. Se. nat. sér. v. t. x. (1869) p. 336. 
$ Heinrich, in Landw, Versuchsst. xiii. 1371, p. 136. 
