EXTERNAL CAUSES OF GROWTH AND FORMATION. I 299 



the rate of growth, and in this respect their influence is ephemeral ; or, secondly, 

 they guide growth and form into definite channels (formative stimuli), in so far 

 as they influence either only the number and size, or also the position, symmetry, 

 direction, or polarity of organs. The results produced are in individual cases 

 either quantitative only or, in more complex cases, qualitative as well. 



Let us begin with the consideration of the influence of ether vibrations, and 

 of these we need consider here only heat and light, for electricity plays no part in 

 determining growth and form in plants. At the very outset we become conscious 

 of the fact that growth, and indeed all the vital processes in the plant, take place 

 only within certain limits of temperature, and the phenomena of everyday life 

 prove to us that these limits are most varied amongst different plants a fact 

 which is confirmed by scientific research. Just as in other functions we discover 

 that there are three cardinal points in temperature for growth, a minimum, 

 a maximum, and an intermediate optimum, the data with regard to which we 

 may take from PFEFFER (Phys. II). Arctic Algae appear to be able to grow 

 at temperatures under o C., but the minimum temperature for most fresh- 

 water Algae lies about o C., or slightly higher. Among higher plants seeds 

 of Triticum vulgare and Sinapis begin to germinate just above o, while 

 Phaseolus commences its development at 9, Cucumis sativus at about 16, 

 the bacillus of tuberculosis at 30, and the thermophilous Bacteria at 

 temperatures even higher still. The maximum temperature for many marine 

 Algae is especially low ; there are, however, no data on record as to the 

 maxima which Algae inhabiting arctic seas can tolerate. The very low 

 maximum of 16 is recorded for Hydrurus (a fresh- water alga), while that 

 of the majority of land plants lies somewhat between 30 and 45 ; it is 

 only in the case of succulents that growth may take place at a temperature of 

 from 50 to 52 C. (compare p. 44). On the other hand, thermophilous Bac- 

 teria can flourish in media which, owing to fermentative processes, exhibit 

 a maximum temperature of 75, and certain Algae can endure temperatures 

 even higher than that, e. g. in the neighbourhood of natural hot springs. Lo WEN- 

 STEIN (1903, Ber. d. bot. Gesell. 21, 317) finds, however, that the Algae in the 

 Karlsbad springs cannot endure a temperature above 52 C. In general terms 

 it may be said that in the case of plants inhabiting cold regions both cardinal 

 points of temperature are low, while in the case of those accustomed to warm 

 surroundings, and also of parasites in warm-blooded animals, these points stand 

 high ; the range of temperature between these maxima is very considerable, 

 ranging as it does from 16 in the case of Hydrurus, between 30 to 40 for the 

 majority of plants, up to 50 or even more in many Cactaceae. As may 

 easily be understood, the requirements of the plant as regards heat, indicated 

 by the position of the minimum and maximum cardinal points, is a factor of 

 fundamental importance in determining the distribution of plants on the earth's 

 surface. The position of these points is by no means fixed, since they may be 

 altered both in higher plants and also, and more especially so, in Bacteria. Thus 

 DiEUDONNfe (cited by PFEFFER, Phys. II, 91) found that he was able, by gradual 

 acclimatization, to get Bacillus anthracis to endure a minimum of 10 instead 

 of I2-I4, and Bacillus fluorescens to withstand a maximum of 41-5, in place 

 of the normal 35. It has also been shown that the position of the cardinal points 

 often depends on other factors, such as food, oxygen, light, &c. (compare PFEFFER, 

 Phys. II, 91). In addition to specific differences there are also differences in the 

 case of individual organs and various developmental stages. For example, the 

 flowers of many of our spring plants develop at a much lower temperature than 

 the vegetative organs, which begin to appear after the flowering period is over 

 (TussUago, Crocus, cherry, &c.). Germination of spores in Penicillium takes 

 place between 1-5 and 43 C., the further growth of the mycelium between 2-5 

 and 40 C., and the formation of spores only between 3 and 40 C. (WIESNER, 



