398 PHYSIOLOGY OF GROWTH. 



branch, making the experiment at the beginning of April. The 

 turns of the spiral must be as close together as possible, and the 

 ends of the string firmly tied together. Diminution of pressure 

 we produce by splitting the cortex and bast of a three-year-old 

 branch for a distance of about 3 cm. by six equidistant radial 

 longitudinal cuts. If we examine the branches in August we 

 shall find that the diameter of the ligatured branch under the 

 ligature is considerably less than above or below it, while, on the 

 other hand, the reduction of pressure has induced a not incon- 

 siderable acceleration of growth in thickness in the part of the 

 branch subjected to it. For experiments on the effects of increased 

 pressure on the growth in thickness of twigs, I employed, with 

 very good results, Salix cinerea. The ligature was fixed at the 

 commencement of April, and removed at the commencement of 

 August. The region under the string was, at the end of the ex- 

 periment, much thinner than the parts of the branch above and 

 below. 3 



1 See Scholtz, Cohn's Beitrdge zar Biologie der Pflanzen, Bd. 4. 



2 See Hegler, the same, Bd. 6. 



3 Exhaustive investigations into the anatomical structure of the wood pro- 

 duced when the pressure is artificially increased or diminished have been made 

 by H. de Vries. See Flora, 1872, No. 16, and 1875, No. 7. 



162. Influence of Temperature on Growth. 



It is a well-known fact that in all plant structures growth by no 

 means proceeds at the same rate at different temperatures. 

 Growth is only exhibited within particular limits of temperature. 

 At certain lower and higher temperatures growth completely 

 ceases, and to prove this for lower temperatures we conduct the 

 following experiment: A well-developed pea seedling, which has 

 been grown in loose, moist sawdust until its main root has attained 

 a length of 3-4 cm., is placed in the apparatus depicted in Fig. 126, 

 together with some moist cotton wool. I found, e.g., that at a tem- 

 perature of 20 C. the growth of the root, in the course of eight 

 hours, was 5 mm. I now left the apparatus for twenty-two hours 

 in an unheated room, at a temperature of l-2 C. At the end of 

 this time no perceptible growth could be determined. Growth was, 

 however, observable (10mm.) after the apparatus had been left 

 for the next eighteen hours in a room at a temperature of 15 C. 



Accurate researches on the influence of different temperatures 

 on the rate of growth of plants are very tedious, but we must, 



