258 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 30 



growth, reach the cells located at a distance from the main trunk 

 lines, it is necessary for them to pass through the walls of the inter- 

 vening cells. The more permeable these walls are to water and the 

 foods dissolved therein, the better are the chances for rapid growth 

 of the meristematic tissues. Wilted plant cells can not grow. 

 Growth ceases even when the cells just begin to wilt. Cells stop 

 growing when they lose water faster than they can absorb it or when 

 they can not get a sufficient amount because of a blockade in the 

 line of cell Avails connecting them to the water mains (vascular 

 tissue). The entire system is very delicately balanced. 



How is this sap or water system related to phototropic bending? 

 As Professor Priestley states, " In this delicately balanced equilib- 

 rium, strong lateral illumination may mean that the sap supply first 

 fails on the side more directly lit, where evaporation will more 

 rapidly bring about a state of ' incipient drying ' in the walls of the 

 tissues. * * * jf i\^Q walls between the vascular supply and 

 superficial meristem are in this condition, food supplies to the meris- 

 tem will fail, and there will be a cessation of meristematic growth." 

 Less growth will occur on the drier or more illuminated side. This 

 will result in a positively phototropic bending. 



In etiolated shoots the mechanism of bending is quite different. 

 Very little light will cause marked growth curvatures, whereas with 

 shoots previously illuminated a stronger light is necessary. The 

 walls of the cells making up the tissue in etiolated shoots contain fat 

 and protein, a substance similar to egg white. These substances pre- 

 vent the ready passage of sap and water from the vascular supply 

 to the meristematic tissue, which, under favorable conditions, is 

 capable of rapid growth. Blaauw, a Dutch scientist, points out the 

 similarity between light in its photochemical effect on a photo- 

 graphic plate on the one hand, and on an etiolated coleoptile on the 

 other. Relatively small quantities of light produce a photochemi- 

 cal action in these shoots. Protein and fatt}^ materials disappear 

 from the cell walls, the latter substances migrating mainly to the 

 cuticle. The passageway between the meristematic cells and their 

 water and food supply is opened up. In the words of Professor 

 Priestley, " Increased superficial growth now ensues. Growth as a 

 whole may be as active as ever on the more brightly lit side of 

 the etiolated shoot, but it is differently distributed. More cells are 

 added to the surface of the stem and leaf, and less proportionately 

 contributed to the inner layers of the shoot axis. The result is, 

 therefore, in the aggregate, a retardation of growth in length on the 

 illuminated side and a positive phototropic curvature." 



Although many roots are not sensitive to light there are a few, 

 as previously mentioned, which show negative phototropism. The 

 section of the root capable of bending is situated just back of the 



