EXTERNAL CAUSES OF GROWTH AND FORMATION. I 301 



to form. In such cases the formation of ice-crystals must be the cause of death. 

 This is all the more extraordinary seeing that other plants, such as our forest 

 trees and many herbaceous plants which grow during the winter {Stellaria 

 media, Senecio vulgaris), can endure alternate freezing and thawing many times 

 in succession. Frozen members are, however, at length killed by lowering the 

 temperature still further, and no turgescent cell can tolerate indefinite reduction 

 of temperature. Possibly death is due in many cases only to the withdrawal of 

 water in the formation of ice, a withdrawal which plants can tolerate only to acer- 

 tain degree. At least the fact is that certain anhydrous organs, such as seeds and 

 spores, are not killed by being subjected to the very lowest temperatures, e. g. 

 — 200° C. for five days (Brown and Escombe, 1895), or — 250° for a shorter period 

 (Thiselton-Dyer, 1899). [Mez (1905) has shown that many plants do not 

 suffer injury by the withdrawal of water that takes place on the formation of 

 ice, and which always ceases at a temperature of — 6° C, but are killed by lower 

 temperatures. In this case death is due to actual cold, not to loss of water in 

 consequence of low temperature.] To 

 go further into the problems which this 

 subject suggests would carry us too far, 

 so that we may refer for further details 

 to the literature on the subject, and 

 especially to Pfeffer's (Phys. II, §§ 65- 

 68) treatment of the question. 



We have up to now learned that 

 temperature acts as a stimulus on the 

 thermotonic plant, either accelerating, 

 retarding, or altering the rate of growth, 

 and that these effects are most com- 

 monly exhibited during the period of 

 elongation. Notwithstanding differ- 

 ences in the rate of growth, the absolute 

 size and a similar shape may be 

 reached, if only the duration of 

 growth alters correspondingly. This 

 is a phenomenon of quite general oc- 

 currence ; plants growing at optimum 

 temperatures do not show differences in 

 appearance from those which have been 

 cultivated at supra- or infra-optima. Changes begin to appear, however, when 

 the limiting temperatures are approached ; near the minimum the length of the 

 growing region increases, and shortens near the maximum (Popovici, 1900) ; 

 further, the internodes become shorter if the culture be maintained for long at a 

 low temperature, lengthened duration of growth does not affect the neces- 

 sary elongation. Temperature thus induces also a formative result ; such 

 results have been recorded, but they apparently play on the whole only a 

 limited part. Vochting (1902) has recorded a far-reaching effect of altera- 

 tion of temperature in the case of the variety of potatoes known as 'Mar- 

 jolin '. At from 6° to 7° C. tubers arise from the main shoot (Fig. Sy, I), but at 

 20° C. these are replaced by normal leafy shoots (Fig. 87, //). Qualitative 

 differences such as these, however, are only rarely induced by temperature. 



When we turn to light we are dealing with a factor which obviously must 

 always have a very great influence on the formation of the plant (compare p. 251). 

 The effect of light on growth is fundamentally different from that of heat. 

 Many organisms can pass through their entire life-cycle in darkness ; in other 

 cases, at least some parts, e.g. roots, can do without light, but /zg/i^ is generally not 

 a direct condition of growth. Indirectly it is certainly essential to the existence 



Fig. 87. — Tubers of ' Marjolin ' potatoes. /, after 

 cultivation for four to five weeks at a temperature of 

 6-7° C; //. after cultivation for seventeen days at 

 25 C. The roots are not represented in the drawings. 

 V, main shoot, A^ tubers, L, etiolated leafy shoots. 

 After VdcHTiNG (Bot. Ztg., 1902, PI. 3). 



