ON THE MODE OF ACTION OF HEAT ON PLANTS. 



179 



ii. p. 659), in his calculations of the aggregate heat necessary 

 for annual plants in cultivation, has given a table in which the 

 days of sowing wheat, maize, &c, in different countries are 

 given approximative] y from a mean estimate of years and 

 customs. That they have been thus obtained is evident from 

 the circumstance that the dates are almost always the 1st or the 

 15th of a month ; whereas direct observation would have often 

 given some intermediate days. It is, moreover, impossible that 

 germination should everywhere commence within the same 

 period after sowing. When frosts or droughts come on, the 

 seeds do not germinate. If, therefore, M. Boussingault has 

 sometimes arrived at different figures for the aggregate heat 

 required for the same plant, it can be no matter of surprise ; and 

 if he has more frequently obtained figures very near to one 

 another, it may be supposed that different causes of error have 

 compensated each other. I have made experiments of the same 

 nature, to which I shall presently refer. The aggregate heat 

 from the time of sowing to the maturity of the seeds in the same 

 species has never been strictly the same; it has, indeed, often 

 been very different, and I shall presently state the reason. I do 

 not say this to detract from the merits of the method proposed 

 by M. Boussingault — a method which I constantly make use of 

 — but in order to show the improvements of which it is suscep- 

 tible in order to enable us to reap the full benefit of it. 



2. The temperature of the soil must influence the progress of 

 vegetation ; and it is well known that, with relation to the tem- 

 perature of the atmosphere, it follows a curve, diverging more 

 or less according to country, and even according to soil. 



3. All temperatures below 0° (the freezing point) are totally 

 useless to plants, as congelation stops the absorption and circula- 

 tion of liquids. It is also certain that low temperatures, such 

 as +1°, +2°, and +3°, or thereabouts, are not sufficient to excite 

 many of the phenomena of vegetable life. Thus wheat, sown in 

 autumn, will rest stationary during the winter, although the 

 temperature may frequently remain for several days above 0°. 

 Thus, the date-tree in the north of Spain, the Ginkgo in many 

 parts of the centre of Europe, will never flower, although the tem- 

 perature is sufficient for them to develop their leaves and to grow. 

 Many seeds, in certain degrees of temperature, will rot, instead 

 of germinating. The temperature to be taken into account must 

 therefore be only that which is above a certain degree, varying 

 according to the species and to the vegetative phenomenon con- 

 sidered, for that only is the useful temperature. But how shall 

 we ascertain this starting point, so variable according to species 

 and phenomena, and so obscure when we are considering, for 

 instance, the commencement of germination or of flowering ? 



o 2 



