662 SECONDARY GROWTH IN THICKNESS 



trees, such as Tilia, Betula, Pinus sylvestris, etc.) ; occasionally a few 

 chlorophyll corpuscles may also be present. 



Mention has already been made (p. 334) of the ringing experiments 

 carried out by Hartig, and more recently by A. Fischer, 304 which show 

 that the carbohydrates formed in the leaves travel downwards in the 

 stem exclusively in the extra-cambial conducting parenchyma and not 

 in the xylem-parenchyma. One set of Fischer's experiments, in which 

 branches of Primus Avium, Tilia and Betula were ringed in two places, 

 is particularly instructive ; no starch entered the (leafless) region 

 between the two incisions, even after the lapse of several weeks. 

 The upward transportation of carbohydrates in spring-time, on the 

 other hand, takes place mainly in the wood-vessels, as we shall 

 see later on. While, therefore, the xylem-parenchyma no doubt serves 

 in the first instance as a repository of non-nitrogenous plastic materials, 

 it would be going too far to regard it as a storage-tissue pure and 

 simple, or to maintain that it has no carbohydrate-conducting capacity 

 whatever. Like the medullary-ray tissue, the xylem-parenchyma cer- 

 tainly plays some active part in connection with translocation. The 

 prevailing elongated form of its cells, and the relative abundance of pits 

 on their transverse walls, further show that this tissue is not merely 

 concerned with local interchange, but may also be responsible for 

 conduction over greater distances. 



If the cambial xylem-parenchyma mother-cells remain undivided, 

 not infrequent occurrence the resulting permanent elements, while agree 

 ing closely with typical xylem-parenchyma cells as regards the character of 

 their cell-walls and the nature of their contents, differ in their spindle- 

 shaped form (Fig. 275 a). These elements, which frequently accompany 

 or even entirely replace {e.g. in Viscum, Ca/ragana arborescens, Spiraea 

 salicifolia) the typical xylem-parenchyma, are the intermediate cells 

 (Ersatz fasern) of Sanio. Each intermediate cell is homologous with a 

 spindle-shaped group of ordinary xylem-parenchyma elements. The 

 sole difference consists, as stated, in the fact that the cambial mother- 

 cell remains undivided, in the one case, and undergoes transverse septa- 

 tion in the other. Since this distinction is manifestly of no value from 

 the physiological point of view, the difference between septate and 

 unseptate bast-fibres being similarly regarded as unimportant we shall 

 follow the example of Troschel, and extend our conception of xylem- 

 parenchyma so as to include the intermediate cells. 



Having in the preceding paragraphs given some account of the 

 typical elements of the secondary wood, or, in other words, of those which 

 correspond to the normal components of the primary hadrome-strands, 

 we must now turn our attention to the intermediate forms, which, as 

 already stated, effect the transition from the mechanical to the con- 



y 



: 



