1909.] 



Vegetable Assimilation and Resjnratioti. 



35 



a considerable diminution in thickness niiglit take place before much change 

 of area would be effected. 



A uniform close-meshed network of veins, such as occurs in Cercis (and in 

 Lime), also resists shrinkage. In leaves which shrink most, on the other 

 hand, the ultimate ramifications of the veins are more slender and delicate 

 {Nicotiana, Cucurbit a). 



The conditions on August 9 were fairly severe, but Vitis, Cercis, and the 

 Cherry Laurel gave to the eye no sign whatever of decreased turgidity. 

 Yet the Cercis leaves had shrunk on the average nearly 2 per cent., and 

 Cherry Laurel over 1 per cent. Vitis is the most resistant of all the leaves 

 observed. 



The conclusion from all the results is that shrinkage is a general 

 phenomenon, and can occur to the extent of 2 or 3 per cent., or sometimes 

 more, without very obvious signs of flaccidity accompanying it. It is, 

 therefore, highly desirable froni the point of view of the half-leaf dry weight 

 method that shrinkage should be eliminated or corrected for. 



Treatment of Detached, Leaves. 



Greater care to ensure turgidity during experiment might be useful in many cases ; 

 but this is difficult in the sun, as all obsei-vers have noted. The difficulty is especially 

 great with detached leaves, and most of those who have employed them have worked 

 either entirely in the shade or with translucent screens. 



To modify the illumination is, however, not alwa3-s admissible ; but another factor, 

 the water supply, is amenable to control. In cutting leaves from the plant, air is apt to 

 enter and block the cut vessels, and so hinder the passage of water to the lamina. 



Sachs' method of collecting his material in the early morning when there was little 

 transpiration is the most efficient way of avoiding this. 



Brown and Escombe, gathering their leaves later in the day, cut through the petioles 

 under water.* In experimenting with Helianthus tuherosus, however, even leaves so 

 treated were found to droop very rapidly in the sun. If, on the other - hand, freshly 

 boiled distilled water was used, they l emaiued turgid much longer. 



Steaming the cut end of the petiole is sometimes of advantage. - Its success may be 

 due either to the driving of air from the vessels or to the killing of the tissues and the 

 surrounding of the vessels on all sides with available water. The treatment is objection- 

 able, however, since there is danger of secondary injurious effects. Moreover, it is not 

 always more successful than careful cutting under water. 



It is probable that almost the whole difficulty in the treatment of detached 

 leaves is due to the blocking of vessels by air : for in an experiment withi 

 Helianthus annuus, for which leaves were cut from the plants at 5.45 a.m.^ 

 while still wet with dew, the use of freshly boiled distilled water was quite 

 sufticient to ensure, in three leaves out of five, a degree of turgidity even 

 greater than would have existed on the plant itself under similar conditions. 



* Lor. Ci't.. p. 57. 



