THE AXGIOSPERMAE : LEAVES 1033 



disappearance of the lateral traces either by abortion or by fusion with the 

 median trace. Certainly the three-trace type is very widespread, not only in 

 palmate but also in some pinnate leaves. 



Salisbury has show-n that in general terms there is a positive relationship 

 between the amount of xylem developed in a petiole and the size of the leaf 

 blade. The proportion of petiolar xylem is greater in mesophvtic leaves and 

 less in those which show xeromorphic protections, such as hairy coats. It is, 

 therefore, presumably due to differences in the transpiration rates from 

 different leaves. Such a relationship has an obvious physiological importance 

 and it is significant that the leaf controls its ow^n supply channels. Thoday 

 has shown that the control extends beyond the petiole, and includes a region 

 of influence, which is delimited both longitudinally and tangentially, in the 

 vascular zone of the stem. 



In the region just below the node the leaf trace passes inwards obliquely 

 through the cortex and enters the vascular zone, where it may either maintain 

 its independence for several internodes downwards or may unite quickly 

 with the other components of the zone. In the independent part of its course 

 its growth is correlated with the growth of its leaf, as it is in the petiole, 

 and ceases when the leaf is full grown. There is little radial cambial growth 

 and what there is is largely parenchymatous, which may be necessary 

 mechanically, to allow for the outward bending of the trace towards its leaf. 

 The mechanical correlation is probably not perfect, and the withering of 

 lower leaves is usually due to the straining or even the breakage of their trace 

 bundles by secondary growth of the stem wood. The traces of deciduous 

 leaves are, of course, only active in the stem of the current year. In the older 

 stem the trace bundles remain embedded in the woody zone, in which they 

 are completely merged, continuing to grow in harmony with the general 

 growth of the stem, i.e., under apical control. The traces of evergreen leaves 

 remain active for several years, during which secondary- stem wood is formed 

 outside their point of origin, but with this wood they have no connections, 

 nor are thev continued outwards after the death of their leaves, as happens 

 in some Conifers, notably in Araucaria, where traces of old leaves may persist 

 across the secondar}^ wood for many years. 



Leaf Fall. 



The dropping both of deciduous and evergreen leaves at the end of their 

 life period is an active process known as phylloptosis, and is due to the 

 formation of a definite separation zone or abscission layer at the base of 

 the petiole. Without this preparation severance does not take place, as may 

 be seen by the persistence of dead leaves on a withered branch, after all the 

 leaves of other living branches have been shed. 



The primary- phenomenon in leaf fall is the production across the base of 

 the petiole of a separation layer of cells with dense contents, either with or 

 without a renewal of active cell division (Fig. 1025). The middle lamellae of 

 cells in this laver swell and dissolve, thus allowing the cells to separate and 



