NATURE AND SOURCES OF FOOD. 101 



Fig. 15. 



Fig 15. Vertical section through a young branch and petiole, showing the manner 

 in which the vascular and cellular tissues of the leaf communicate with those of the 

 stem, m pith of the stem ; fv nbro-vascular tissue next the pith passing into the 

 petiole which is articulated to the axis ; pc, ^wf parenchyma of the stem ; &, bud in the 

 axil of the leaf; c, cushion or swelling below the leaf; /, forming fracture. 



ex-current layer situated on the upper surface of the petiole 

 and lamina, and which is immediately connected with the 

 alburnum of the stem, and a recurrent layer situated imme- 

 diately beneath the first layer, on the under surface of the 

 petiole and lamina, which is connected with the endophleum or 

 inner fibrous bark. The sap is brought from the albur- 

 num by the ex-current or upper-layer, into the leaf and distri- 

 buted to all parts of its upper surface ; having undergone all 

 those chemical changes which render it suitable for vegetable 

 assimilation, or having been elaborated into proper juice, 

 then conveyed by the recurrent layer of fibres along the under 

 surface of the lamina and petiole into the bark, down which it 

 descends to the roots. 



That the sap re-descends from the leaves to the roots by the 



