202 SECTIONAL ADDRESSES 



It is not easy to obtain further experimental proof of the presence of 

 the osmotic systems connected with each developing bud, because they 

 are so closely connected with processes of growth and differentiation and 

 cease to operate if the experimental procedure prevents further growth. 

 When the young bud is cut across, liquid can be seen to well out from the 

 veins, but it is impossible to collect drops of this liquid which are not 

 mixtures of sap from both wood and phloem. The sap in this differ- 

 entiating wood is certainly much more concentrated than any recorded 

 concentrations for the tracheal sap, but it must be remembered that all 

 analyses have been made upon extracts which are drawn mainly from 

 fully differentiated tracheae. Observers who have tried to distinguish 

 between the sap from the outer and inner tracheae have always found 

 that the sap from the outer and younger contains more solutes. In all 

 cases where sap is collected from differentiating tracheae but little can 

 be obtained, because when these elements are cut open and the sap released 

 the processes of growth and differentiation come to an abrupt end ; the 

 osmotic system thereupon soon ceases to function and no more sap collects. 

 It is possible to collect small supplies of sap from the new tissues differen- 

 tiating over the surface of the old wood in spring, and such samples in 

 both hardwoods and softwoods have shown themselves more concen- 

 trated than a 0-25 M cane sugar solution when tested by Bargers's 

 method. In a drop from the differentiating tissues of the plane, 

 the reducing substances after inversion, determined by the method 

 of Hagedoorn and Jensen, were equivalent to about 2-6 per cent, 

 sucrose. 



Water Movement into the Expanding Foliage. — As the tracheal elements 

 absorb water, as they differentiate in and beneath the bud, naturally 

 their liquid contents are under pressure and are leaking outwards into 

 the leaf tissues ; but as the foliage expands, evaporation from the larger 

 surface rapidly removes any excess of water, and ' weeping ' cannot be 

 detected long after sunrise even in young leaves. But evaporation will 

 still continue during the day, so that the living cells of the leaf tend to 

 withdraw water from the tracheae faster than it enters, with the result that 

 the liquid contents of the tracheae are soon under tension. It is surprising 

 how soon this condition of tension can be detected in the tracheal system 

 of the expanding bud. If the bud is cut across under Indian ink, which 

 contains a fine suspension of carbon particles, and the cut surfaces washed 

 under running water, many of the tracheae will be seen to be injected 

 with the ink. The injected tracheae will be found to be protoxylem 

 elements of medium age. Older collapsed elements are not injected, and 

 younger elements, still differentiating and containing liquid contents 

 under pressure, are also not injected. In a young bud, a freshly cut 

 surface, whilst showing ink in some injected vessels, will also show 

 liquid welling from the bundles for a time because of the excess liquid 

 still accumulating in the elements which were differentiating when the 

 bud was cut across. 



The same phenomenon can be demonstrated in the case of the tracheae 

 differentiating from the base of the bud over the surface of the old wood, 

 the hardwood being naturally a more suitable subject for such experiments. 



