62 THE PLANT CELL. 



readily traced (see Fig. 416). The function of this tissue is to 

 aid in the downward translocation of elaborated food-material, 

 which passes in from the mesophyll through the endoderrais into 

 the transfusion-cell, and so into the phloem.* 



The sieve-areas in the sieve-tube of Pinus are situated, not on 

 the end-wall, but laterally on the radial walls, so that this would 

 seem to facilitate the inward diffusion of elaborated sap from the 

 mesophyll into the phloem. In bifacial leaves elaborated sap 

 passes directly from the cells of the palisade layer and spongy 

 parenchyma into thin-walled phloem cells, situated on the under 

 side of the endings of the leaf-bundles. 



In ultimate function sieve-tubes and their companion-cells act 

 in the translocation and storing of elaborated food-materials, and 

 in addition each tube is possibly concerned in the manufacture or 

 further elaboration of certain of these food -materials. The elabo- 

 rated sap from the mesophyll cells of the leaves finds its way into 

 the phloem of the leaf-traces, and so downwards by means of the 

 perforations in the sieve-plates. These perforations are large, 

 and through them large quantities of sap, food-granules, and cyto- 

 plasm can pass at a time. All the way down the stem and root 

 elaborated sap can, after being perhaps further changed in the sieve- 

 tubes and companion-cells, find its way by osmosis into the cortex 

 externally, and the cambium internally; and in the spring the 

 stored nitrogenous and carbohydrate food in the tubes is converted, 

 by means of enzymes, into soluble proteids and carbohydrates, 

 which pass out laterally by means of osmosis into the tissues 

 requiring fresh elaborated food for the purposes of growth and 

 general nutrition (see also Chap. x.). 



Subsidiary Elements of the Phloem. — These are : — 



(a) Phloem-parenchyma. 



(b) Bast-fibres. 



(a) The phloem-parenchyma is composed of small thin-walled 

 cells lying between the sieve-tubes, and possessing protoplasm and 

 reserve starch. Functionally these cells form a sort of supple- 

 mentary tissue to the sieve-tubes, and are useful in the storage 

 of carbohydrates. 



* Transfusion-cells exist also in the endodermis of the roots of Iris ; and 

 in Pinus leaf the transfusion-cells on the xylem side of the bundles permit 

 of the passage of water from the wood into the mesophyll. 



