K.— BOTANY 20I 



separate cells) with the liquid in the vessel in the leafy shoot, from which 

 it is separated at most by a very occasional unperforated but permeable 

 wall. Naturally, then, the water withdrawn from the old wood at the 

 lower end of the system, by the differentiating elements which are there 

 being added to the vessel system, is driven forward under pressure into 

 the distal end of the tracheal system, where it is slowly released into the 

 still growing tissues of the leafy shoot. 



The facts of anatomy and development are the clearest evidence that 

 each differentiating vessel, common to both leafy shoot and woody axis, 

 must thus transfer water from the woody axis to the growing shoot. The 

 growth of the bud, especially the vigorous cell expansion in the growing 

 tissues, is clear evidence that water is thus moving from the woody axis 

 to the young shoot. The following considerations support the view that 

 this water movement takes place under the impulse of a mechanism that 

 is actuated by the growth and differentiation which begins in the bud 

 itself and spreads from thence to the axis. 



Occasionally so much sap is driven into the vascular system of the 

 young developing leaves that they ' weep ' from the tips of the veins ; 

 the sap flows out of the veins, injecting the intercellular spaces in which 

 it accumulates until it flows out through the stomata on the teeth near 

 the termination of the veins. Such ' weeping ' is often spoken of as 

 caused by root pressure, as it certainly is in the case of seedlings, but in 

 the tree the connection with root pressure is very indirect. So long as 

 root activity throughout the winter has accumulated sufficient water in 

 the old wood, weeping may occur from the buds. It can be demonstrated 

 in buds on twigs removed from the tree provided they are kept in warm 

 saturated air. Th. Hartig has pointed out, in Carpinus Betulus particu- 

 larly, a tree which shows both ' bleeding ' from the cut stump and 

 ' weeping ' from buds in the intact tree, that the two processes do not 

 synchronise. In a particular season, ' bleeding ' from a cut stump 

 began on February 22, but no buds were observed to ' weep ' until 

 March 17. ' Bleeding ' occurred from g A.M. till midday each day, but 

 ' weeping ' began in the afternoon, was strongest at night and ceased 

 about one hour after sunrise. The sap flow from the veins of the leaves 

 in the buds, therefore, does not synchronise with the time of highest sap 

 pressure, derived from root activity. 



Many points about bud development in the tree become much clearer 

 when it is recognised that the movement of sap in these developing 

 tissues depends so directly upon processes originating in the bud itself. 

 All buds on the tree do not start into activity at the same moment ; well- 

 developed buds in full sunlight open first, and as they draw off the supplies 

 of water from the wood immediately adjacent we can understand why it is 

 (i) that the buds immediately below the vigorously growing terminal 

 bud of a poplar shoot remain dormant (p. 192), and (2) that the lower 

 shaded branches on which the buds do not commence growth so soon, 

 may never start into growth at all, unless the upper, vigorously growing 

 buds are cut off by man or by a frost ; and that such branches, if the buds 

 do not commence growth, lose water to the differentiating tissues in the 

 main axis. 



H 2 



