xm, c, 6 Brown and Trelease: Shrinkage and Elongation 359 



growth, running parallel to its course and being determined in greater 

 part by identical causes. 



That the water balance is actually decreased at night and increased 

 by day has been found by Mrs. E. B. Shreve, 8 who says of a cylindropuntia : 



"It was found, under conditions of average transpiration, such as occur 

 in the greenhouse in summer, that the water intake at night is less than 

 the outgo, while during the day the intake is greater than or at least 

 equal to the outgo .... An examination of the water-content of stems 

 from plants in the open and from the greenhouse showed that the highest 

 water-content is at 5 p. m. after the close of a bright day, and the lowest 

 just before daylight the next morning, with an intermediate amount at 

 noon." 



In the case of Oestrum the shortening during the day would 

 appear to be very evidently connected with excessive trans- 

 piration. 



That excessive transpiration may check growth or cause ac- 

 tual shrinkage has been demonstrated by Lloyd 9 in Eriogonum. 

 Concerning his experiments he says: 



The daily march of growth is as follows: During the early daylight 

 hours until about 8 there is usually a slight rise in growth-rate. After 

 that hour the rate falls to a low value, or, much more frequently, there 

 ensues an actual shrinkage. This is the period during which the loss 

 of water by transpiration is rapidly increasing, reaching its maximum 

 at about noon. Coincidentally with the checking of transpiration, the 

 growth-rates rapidly increase in value, the maximum rate being attained 

 by 1 or 2 p. m., and thereafter maintained, with fluctuations, until 6 p. 

 m., when the rates fall to the night values. The afternoon rates are great 

 enough to more than make up for the negative behavior of the morning, 

 except, as above stated, under unusual conditions. 



* * * it was found possible experimentally to alter the rates both 

 positively and negatively quite independently of the constancy, increase, 

 or decrease of illumination, even when this has been increased with 

 respect to the growing part by insolation from three directions. There 

 seems, indeed, to be no maximum insolation normally occurring in the 

 field at this locality which can cause any cessation or inhibition of growth 

 when conditions obtain which insure water-supply to the growing part. 

 Thus when a cessation of growth is apparent, it may be checked, and 

 high rates instituted, by the removal of leaves (which divert the water- 

 supply), by increasing the vapor-tension in the vicinity of the growing 

 part, * * *. 



The present results on Cestrum indicate that a retardation 

 or inhibition of growth during the day may be due to other 

 conditions than the direct action of light. In this they agree 

 with Lloyd's results and tend to support MacDougal s earlier 

 conclusions : 10 



•Carnegie Inst. Wash. Year Book 13 (1914) 98, 99. 

 'Carnegie Inst. Wash. Year Book 15 (1916) 58. 



-MacDoueal D. T„ Influence of light and darkness on growth and 

 development Mem. N. Y. Bot. Garden 2 (1903) 307. 



