Some Hydration Reaxitions and Growth. 155 



rate and course of growth of any plant with differentiated tissues which 

 does not take into account the mechanical composition of the organs, 

 and especially the arrangement of the growing cell-masses with respect 

 to mature parts, may encounter many pitfalls and can hardly fail to 

 be inadequate. 



' \er<>^: 



s 



IS 

 25 

 35 

 45 

 55 

 65 

 75 

 85 

 95 



Feb.ZI 

 5 p.m. 



Feb.22 

 Ba.m. 



X5 



Fig. 39. — ^The upper part of the figure shows the course of elongation of a stem of Helianfhus 

 annuus for 24 hours beginning at 5 p. m., with temperatiires of the plant as indicated, X 5. 

 Increase in length denoted by dovmward movement of the pen. Shown on a scale of milli- 

 meters as indicated, the total elongation during the period being 19 mm. The lower tracing 

 shows variations in thickness of a stem of Helianthus 15 cm. from apex, the increase being 

 denoted by the upward movement of the pen, with temperatures of the plant as indicated. 

 Shrinkage or cessation of enlargement began after midday, but increase was again mani- 

 fested by evening. The variation is amplified 30 times and is shown on a millimeter scale, 

 the actual increase during six days being about 0.6 mm. 



Opportunity for the measurement of growth in another type of 

 structure was presented by the legumes of Phaseolus cultivated in the 

 glass-house of the Desert Laboratory in April 1918. These pods are 

 first measiu"able when they have attained a length of about 3 cm. and 

 a thickness of 2 mm., and as they attain a final length of 10 to 12 cm. 

 in a week, the rate is rapid enough to afford ready means of detecting 

 variations and connecting them with possible modifying agencies. 

 The thickness of a mature pod through a full-sized bean may be as 

 much as 6 to 8 mm. The imbibition or swelling capacity of the entire 

 structure and .its contents was tested at two different stages. The 

 measurements of this capacity in the earUer stage was made upon 

 sections of the pod less than a centimeter in length, which, by their 

 bulging contour, showed the presence of an embryo bean inside, al- 

 though this was much less than a millimeter in diameter and probably 

 played a very small part in the sweUing. The increases of such sec- 

 tions at two different temperatures were as noted in table 116, the 

 average thickness of the trios of sections being 2.7 to 2.8 mm. 



