656 SECONDARY GROWTH IN THICKNESS 



We must likewise reject the names " soft bast " and " bast-parenchyma," 

 since the former is properly applicable only to the non-mechanical com- 

 ponents of the secondary phloem, while the other should be employed 

 to designate the conducting parenchyma exclusive of the medullary 

 rays in the same region. 



The various tissues that are comprised in the general category of 

 secondary phloem are usually arranged in more or less distinct tangential 

 layers, traversed radially by the medullary rays, which maintain the . 

 interchange of material between the woody cylinder and the extra- 

 cambial region. 



In secondary phloem, as elsewhere, the leptome is made up of sieve- 

 tubes and of companion-cells or of the rows of " albuminous cells," 

 which take the place of companion-cells in Gymnosperms. It also 

 includes some " cambiform " cells ; but these are neither so numerous 

 nor so typical as they are in the primary leptome, and are far less con- 

 spicuously developed than the ordinary leptome-parenchyma (conducting 

 parenchyma). The sieve-tubes and companion-cells, on the one hand, 

 and the conducting parenchyma on the other, are usually grouped in 

 alternating tangential layers. The sieve-tube segments, being derived 

 from cambial cells, have oblique end-walls, each sloping face bearing 

 several superimposed sieve-plates ; as has already been explained (p. 306), 

 this is a feature which facilitates the translocation of protein compounds. 

 The cells of the conducting parenchyma generally contain some chloro- 

 phyll, and at the end of summer become filled with large quantities of 

 starch. This starch, however, disappears again completely in late 

 autumn, before the period of " hibernation " begins ; according to 

 Russow, A. Fischer and others, it is mainly transformed into fat, to 

 some extent also into glucose, and perhaps also into a third as yet 

 unidentified compound, while a fourth and final portion in all proba- 

 bility passes into the woody cylinder, through the medullary rays. The 

 first change in spring is the reappearance of starch ; later this is trans- 

 formed into soluble carbohydrates, which are conveyed to the opening 

 buds along the secondary wood. The "ringing" experiments performed by 

 Hartig, Hanstein and A. Fischer, show that the carbohydrates produced 

 in the leaves travel downwards through the stem in the conducting 

 parenchyma of the phloem, and thence pass into the woody cylinder 

 along the medullary rays. Schellenberg states that, in some woody 

 stems (Vitis, Alnus, Acscuhis, Bdula, etc.), the walls of the leptome- 

 parenchyma cells become more or less extensively thickened during 

 winter, but undergo partial solution in spring ; the thickening layers 

 involved consist of hemicclluloscs. 



The individual sieve-tubes, and their associated companion-cells or 

 albuminous cells only remain functional for a comparatively short 



