468 



SCIENCE 



[N. S. Vol. XLI. No. 1056 



With regard to the existence of a sharply defined 

 reaction of this kind, practically nothing can be 

 deduced from literature references, at least the 

 general opinion about the influence of light on 

 growth is completely at variance with these facts. 

 In the first place so far as concerns the positive 

 or negative influence of light, the general con- 

 ception, supported by numerous facts, is that light 

 exercises a retarding influence on growth. 



Blaauw's results wHch are described in the 

 paper mentioned above and in a later paper^ 

 confirm my original thesis that light does not 

 exercise a flat or invariable effect on growth. 

 Furthermore Blaauw's beautifully arranged 

 experiments by which sporangiophores of the 

 mould were exposed to illumination from four 

 or eight sides, with controlled intensities, 

 demonstrate that the first reaction of this or- 

 gan to a sudden illumination is an accelerated 

 rate of growth, followed by a gradual decrease 

 from which a recovery is made to the original 

 rate. It is to be seen that the general mode of 

 change is similar to that of massive or- 

 gans deprived of light as described above. 

 American reviewers seem to have been equally 

 ignorant of my earlier experiments, which had 

 the force of rendering the older generaliza- 

 tion invalid. Furthermore the indirect effect 

 of light in conditioning differentiations of 

 tissues and thus affecting growth-elongations 

 was pointed out. Blaauw has made an impor- 

 tant contribution by his experimental analysis 

 of the action of light on such simple struc- 

 tures as the sporangiophore of a mould. 



The elongation or enlargement of a cell or 

 of any structure like that of the sporangio- 

 phore of Phy corny ces may be taken as the ex- 

 pression of inequality between the extensibil- 

 ity of the cell material, and its membrane, and 

 of some internal expanding or stretching force. 

 The osmotic pressure of the contents of the 

 vacuoles, or of solutions filling the proto- 

 plasmic interstices has hitherto been relied 

 upon to furnish the necessary force of growth. 



Borowikov has recently established a parallel 

 between the grovrth of certain seedlings in 

 known definite solutions and the hydratation 

 of colloids in the same solutions. This author 

 is therefore led to believe that the stretching 



3 "Light und Wachstum," Zeitschr. f. Botamk., 

 Hft. 8, 1914. 



force of growth is not osmotic but hydratation 

 pressure, and he relegates osmotic pressure, 

 turgidity and its corollaries to an inconse- 

 quential place in the entire matter.* 



Several features of the growth and hydrata- 

 tion of cacti are not without importance in 

 connection with any consideration of this 

 matter. The researches of Eichards and of 

 Spoehr at the Desert Laboratory show that 

 the acidity (malic and oxalic) of the sap of 

 cylindropuntias and platypuntias decreases 

 from its maximum at daybreak to a minimum 

 at about 4 p.m. in the open. The decrease has 

 been shown to be due to the conjoint disinte- 

 grating action of temperature and chiefly of 

 light. The calibrations made by Mr. E. H. 

 Long (paper now in press) brought out the 

 fact that if small cylinders were cut from the 

 bodies of these cacti in series beginning at 

 daybreak and extending to the period of mini- 

 mum acidity, the hydratation capacity of the 

 pieces increases independently of osmotic pres- 

 sure throughout the day and is greatest in 

 those which have been taken from the plant at 

 the time when collateral tests would indicate 

 the lowest acidity. 



Extensive auxanometric records of Opuntia 

 Blakeana made chiefly in March and April 

 show that the growth of the enlarging joints 

 is at a minimum in the morning, with a rapid 

 acceleration parallel with the rising tempera- 

 ture of the open, reaching a maximum about 

 noon and then decreasing to a minimum before 

 3 P.M. The curves of decreasing acidity and 

 increasing hydratation capacity are sym- 

 metrical through the range of acidity from 

 N/10 to N/20 according to available data ob- 

 tained from these plants, and would probably 

 sustain a similar relation in weaker solutions 

 if the acidity were reduced still further. 



From the records cited above however it is 

 to be seen that the acceleration of the rate of 

 growth does not follow that of hydratation to 

 its customary daily maximum. Whether this 

 divergence is due to a shrinkage following a 

 heightened water-loss is not yet known. An 

 ample supply was available to the absorbing 



* Borowikow, ' ' Ueber die Ursaehen des Wach- 

 stums der Pflanzen," Biodhem. Zeitsohrift, 48: 

 pp. 230-46, 1913. 



