194 



LECTURE XII. 



and it has been established that the seeds of cereals, for example, can still germinate 

 — i. e. develope their roots and seedling shoots — at a temperature very near to 

 the freezing-point of water ; whereas the ]\Iaize and the Kidney-bean [Phaseolus 

 vmltifloriis) require at least a temperature of about 9° C. ; and Date-stones, as it 

 appears, only germinate at 15° C. The optimum temperatures at which the quickest 

 growth of the organs of the seedling results, appears in our cereals to be 28° or 29'' 

 C. ; in the other plants mentioned, however, it seems to lie above 30° C. At a 

 continued temperature above 40° C. the germination of all these plants is abnormal, 

 or they perish. 



Of the other most general vital phenomena of the plant, may be men- 

 tioned, in the first place, the streaming of the protoplasm. In the hairs of 

 the Gourd-plant this movement appears only to begin when the temperature 

 reaches 10-11° C; the optimum temperature of the streaming of the protoplasm 

 may lie between 30° and 40° C. ; and towards 50° it decreases more and more, to 

 cease entirely if the exposure continues sufficiently long. 



The relations which we have so far explained between the vegetative processes 

 and the temperature may also be graphically represented. If we suppose a straight 

 line drawn horizontally, and its length divided into a number of equal parts, which, 

 proceeding from left to right, are marked like a thermometer scale, 0°, 1°, 2", 3°, and 

 so forth to about 50°; and if we then further suppose the growth in length of roots 

 or shoots attained at the temperatures named, and in equal times, to be denoted by 

 lines of corresponding lengths, erected perpendicularly on our horizontal line (the 

 so-called abscissa) at the places where the temperatures belonging to it are recorded, 

 these vertical lines represent ordinates of a curve obtained by connecting their upper 

 ends together by a continuous line. It is clear that this line, usually curved, as al- 

 ready said, attains its highest point above the optimum temperature of the abscissa 

 line, and from thence again sinks down to the latter. This curved line we term 

 simply the temperature curve of the growth in length; and to any one at all 

 familiar with these matters, such a curve presents the easiest guide to the relations 

 of law between temperature and growth established by investigation. In like 

 manner the velocity of the protoplasmic movements may of course also be repre- 

 sented by vertical ordinates on a temperature abscissa, and thus in the form of a 

 curve ; and, in general, each function of the plant dependent upon the temperature 

 may be so represented. 



So far as we at present understand the dependence of growth, of the move- 

 ments of protoplasm, the formation of chlorophyll, assimilation, and of the various 

 other irritabilities upon temperature, light, electricity, and external influences in 

 general, all these relationships may be likewise expressed in the form of curves ; 

 in each case the external influences being registered according to their intensity 

 on an abscissa line, and the corresponding efi^ects on the plant being expressed by 

 ordinates. Everywhere, so far as sufficiently strict investigations are to hand, 

 the curves of function so constructed show the main property of temperature 



ascending from the abscissa and returning to it may also be employed for other relations of 

 dependence of the plant upon external influences, was further established subsequently by myself 

 and other observers. This law is, however, expressed generally for the first time in the present 

 lecture. 



