328 CONDUCTING SYSTEM 



to be the first demonstrable nitrogenous product of synthesis, follows 

 practically the same path as the protein-compounds. 



Latex-tubes occupy a special position, since they take part in the 

 conduction of both classes of plastic compounds, besides often acting as 

 repositories of waste-products. 



1. Conducting parenchyma . 



The category of conducting parenchyma (Leit r parcncliyni) comprises 

 a variety of parenchymatous tissues, which all perform essentially 

 similar functions ; the parenchymatous bundle-sheaths and the nerve- 

 parenchyma of leaf-blades, the bulk of the cortical parenchyma of the 

 petiole and stem and the parenchymatous constituents of the primary 

 leptome- and hadrome-strands all come under this heading. Physio- 

 logically considered, these various parenchymatous tissues collectively 

 constitute a definite tissue, inasmuch as they all serve principally for 

 the conduction of non-nitrogenous plastic substances in general, and of 

 carbohydrates (hexoses, transitory starch) in particular. 165 



Conducting parenchyma is always composed of thin-walled, more 

 or less elongated elements; where it forms part of a hadrome strand, the 

 walls are generally lignified. The cells contain living protoplasts, and 

 their transverse walls are usually furnished with simple pits for greater 

 ease of diffusion. In Dicotyledonous leaves the cells of the parenchy- 

 matous bundle-sheaths not infrequently develop lateral branches, which 

 extend toward the adjoining elements of the intermediary tissue or 

 spongy parenchyma. In addition to protoplasm, conducting paren- 

 chyma cells usually contain various sugars dissolved in the cell-sap, 

 or fine-grained transitory starch. Very often the cells in question are 

 also provided with chloroplasts ; their photosynthetic activity is how- 

 ever, in all probability, never very great, and is always subordinated to 

 their principal function of conduction. A totally different subsidiary 

 function namely, storage may assume greater importance ; during 

 the resting season, in fact, when translocation is largely in abeyance, 

 storage often becomes the principal function of the conducting paren- 

 chyma. 



2. L&ptome-parenchyma (eambi/orm cells). 



Leptome-parenchyma consists of elongated thin-walled cells, pointed 

 at either end or attenuated in prosenchymatous fashion, and provided 

 with more or less well-developed protoplasts (Fig. 136). In their 

 typical form these cells closely resemble the cambial or procambial 

 elements from which they are derived ; it is on account of this resem- 

 blance that they have been termed cambiform cells. They are well 

 provided with pits, which are especially numerous on the terminal 



