382 CONDUCTING SYSTEM 



structure form a most promising field for anatomico-physiological 

 research. Westermaier has made a careful physiological study 

 of the medullary vascular system in the Begoniaceae. In this 

 Natural Order, the medullary strands are for the most part cauline 

 bundles of secondary origin. At the nodes they curve outwards and 

 unite at random with traces belonging to leaves which are inserted 

 higher up. Their physiological significance is clearly indicated by the 

 circumstance, that their occurrence is restricted to those Begoniaceae 

 that hibernate by means of tubers or rhizomes {Begonia boliviensis, 



B. tubereulata, B. hybrida, B. ignea, B. rdbusta, B. Rex, B. hydrocoty- 

 lifolia, etc.). In such plants the quantity of translocated materials 

 passing through the stem is far greater than in the case of species 

 with perennial woody shoots. The increased demands made upon the 

 conducting system, in consequence of the alternate collection and redis- 

 tribution of reserve-materials, are met by the development of accessory 

 bundles in the pith. 



Another interesting case that has been elucidated by Westermaier, is 

 that of certain species of Campanula (such as C. Trachelium and 



C. multijiora), which are distinguished by the circumstance that their 

 flowers are very numerous and at the same time often closely crowded 

 together. The portion of the axis below the fascicular or capitate 

 inflorescence has to satisfy the needs of a vast number of seeds, all 

 ripening more or less simultaneously, and must hence be provided with 

 a larger amount of conducting tissue, than is required by the stems of 

 related species in which the flowers are fewer in number, or at any rate 

 more widely scattered and put forth successively. In the former 

 case, accordingly, the ordinary conducting system is augmented by 

 medullary strands. In Campanula Trachelium these accessory strands 

 lie in close proximity to the inner side of the normal woody cylinder ; 

 in C. multijiora they are arranged in two concentric circles, the outer 

 lying close to the circle of ordinary bundles, while the much smaller 

 inner circle encloses the central portion of the pith. 



The presence of cortical bundles can also be referred to physio- 

 logical causes in certain cases. Wherever, namely, the cortical paren- 

 chyma develops an unusually large amount of chlorophyll and generally 

 approximates more or less to the condition of typical photosynthetic 

 tissue, there cortical vascular bundles also tend to appear ; these acces- 

 sory strands evidently serve to supply this photosynthetic tissue with 

 water and mineral salts, and to remove the whole or part of the plastic 

 materials manufactured in its cells. De Bary was the first to put 

 forward this explanation of the occurrence of a cortical vascular 

 reticulum in the leafy marginal expansions of the so-called " winged " 

 stems of species of Latliyrus and other plants ; the same author has 



