666 SECONDARY 'GROWTH IN THICKNESS 



Cell Form. Principal Function. Subsidiary Function. 



4. Thick - walled xylem - paren- Storage and con- Mechanical sup- 



chyrna. duct ion of car- port. 



bohydrates. 



5. Intermediate cells with narrow Do. Do. 



pits. 



6. Thin - walled xylem - paren- Do. 



chyina. 



The problem as to the mode of evolution of the two anatomico- 

 physiological transition series which have just been described, is no 

 longer capable of exact solution. It is probable that, at the time when 

 the Higher Plants acquired the property of secondary growth in thick- 

 ness, primary differentiation had already led to the production of distinct 

 elements serving, as the case might be, for mechanical support, for the 

 conduction of water, or for the translocation and storage of plastic 

 materials. It therefore seems permissible to assume, that these three 

 principal components were represented in the secondary wood at the 

 first inception of secondary growth in thickness, and that subsequently 

 mechanical elements, where needful, undertook the work of conduction, 

 and vice versd, the change of function being attended by a corresponding 

 modification of structure. 355 



The above general discussion of the longitudinally orientated 

 constituents of the woody cylinder, may be supplemented by a few 

 remarks concerning certain features which are restricted to particular 

 families. In the Papayaceae, the wood contains a well-developed 

 network of latex-tubes, which extends throughout the xylem-parenchyma, 

 and also comes directly into contact with the vascular tissue. As 

 regards the occurrence of secretory or excretory organs in the woody 

 cylinder, it may be noted that resin-passages are to be found in the 

 xylem-parenchyma of Conifers, while crystal-sacs occupy a similar 

 position in certain Leguminosae (e.g. Haematoxylon), in Vitis, and in 

 other instances. 



In the vast majority of cases, the tissue composing the xylem-rays 

 is of the nature of conducting parenchyma. Its individual cells are as 

 a rule radially elongated, in accordance with the fact that the medullary 

 rays are specially destined to maintain an interchange of material in 

 the radial direction. The cell-walls resemble those of the xylem- 

 parenchyma, being more or less thickened and lignified, and furnished 

 with very numerous pits on their transverse walls. 



Among the Abietineae, and especially in the genus Pinus, the 

 parenchymatous cells of the medullary rays are accompanied by 

 elements which agree with ordinary tracheides, both in structure and 

 in function (Pig. 276, mt). The walls of these tracheidal cells are 



