EFFECT OF TEMPERATURE ON GROWTH, 
829 
in Sect. 7 that the life of a plant generally and its growth in particular is carried on 
only within certain limits of temperature (in general between zero and 50° C), and 
that each function has apparently in every plant its inferior and superior limits ; 
so that, for example, the lowest temperature at which a plan.t of Wheat can grow is 
different from the lowest at which a Gourd can grow, &c. It has also been shown 
that growth, like other phenomena, is more active the higher the (constant) tem- 
perature above the inferior limit, but that there is a certain temperature at which 
growth reaches its maximum activity, and above which any further rise of temper- 
ature causes a diminution of its rapidity. There is not, in the mathematical sense 
.of the term, any proportion between the rapidity of growth and the height of the 
temperature, and the more accurately the relation between the two has been investi- 
gated, the more difficult is it to express this relation by any mathematical formula. 
It cannot, on the other hand, be doubted that it is of the utmost importance for 
any future theory of the mechanical laws of growth to ascertain the extent to which 
growth depends on temperature, at least in a few particular cases. 
The difficulties of investigations of this kind are however much greater than is 
generally thought ; and the results obtained hitherto, valuable as they are, go no 
further than what is stated above, and give us no deeper insight into the way in 
which that particular mode of motion of the molecules which we call heat is con- 
nected with that mode of motion which causes growth. 
Restricting ourselves to the results at present obtained, it will be seen that they 
have a great practical value in addition to their theoretical significance. A know- 
ledge of the cardinal points of temperature, viz. its superior and inferior limits and 
the particular temperature at which the maximum of action takes place, is indis- 
pensable to investigations of various kinds, in order to get at a correct interpretation 
of the phenomena. On this account a few of the more trustworthy observations 
may be given here. 
In order to determine the cardinal points of temperature to which allusion has been 
made, observations are of value only when conducted at nearly constant temperatures ; 
the means deduced from very variable temperatures may, as I have shown, lead to 
very erroneous conclusions. It is however by no means easy to maintain a sufficiently 
constant temperature for a whole day even by artificial heating or cooling. Special 
difficulty is met with in the determination of the inferior limit or specific zero, since the 
observation must extend over a considerable time — in the case of germination, several 
weeks — to be certain that growth does not take place. It would be possible, by means 
of the apparatus already descrilped, to determine in the course of a few hours whether 
growth still takes place in an internode at a very high or at a very low temperature, and 
at what temperature it is the most rapid, if it were not extremely difficult to regulate the 
temperature of the plant in the apparatus with sufficient exactness. The auxanometer 
will however be very useful even in this case. The observations on this point hitherto 
made, at least those which have any physiological value, have been on germinating seeds, 
as the temperature and moisture of the soil in which they grow can be more easily 
regulated than of the air in the case of internodes. Special facilities are offered by the 
roots of seedlings, as they do not emerge from the soil, and are more easily measured, 
from their simpler and more regular form. The following figures refer only to the roots 
of seedlings, the hypocotyledonary portion of the stem being also, in the case of Dicoty- 
ledons, included in the root. That exactly the same figures are not always obtained by 
different observers is the result of diff'erences in the mode of observation, the amount of 
water, the nature of the soil, the inaccuracy of thermometers, &c. 
