674 SECONDARY GROWTH IN THICKNESS 



roots is entirely dependent upon the close association of living paren- 

 chyma cells with the vascular elements. When root-pressure is active 

 in spring, the sugar which arises from the starch stored in the xylem- 

 parenchyma and medullary rays, may diffuse into the water-conducting 

 elements and then travel rapidly in the transpiration-current towards 

 the growing organs. It has long been well known that the sap of 

 certain trees contains considerable quantities of sugar in spring-time. 

 The sugar-content of the vernal sap is 3*57 per cent, in the case 

 of Acer saccharinum, according to Clark, and 1 - 1 5-3*44 per cent, 

 in that of A. platanoidcs, according to Schroder. 359 This sugar can 

 only be derived from the xylem-parenchyma and the medullary ray 

 tissue, in both of which starch is stored during winter. In summer- 

 time, water, with mineral salts in solution, is pumped by the 

 parenchyma of the absorbing roots into the conducting channels, 

 along which it travels (as the so-called transpiration-current) into 

 the photosynthetic organs ; in precisely analogous fashion soluble 

 plastic materials are forcibly transferred in the spring from the xylem- 

 parenchyma and the medullary rays to the water-conducting elements, 

 and are thus conveyed to the developing leaves and floral organs much 

 more rapidly than could be done by osmosis alone. 



While the transportation of plastic substances is evidently a sub- 

 sidiary function of the water-conducting system, it is nevertheless of great 

 physiological importance, because it renders possible the very rapid unfold- 

 ing of foliar and floral buds that takes place in spring. The correctness 

 of this view which had already been formulated in the first [German] 

 edition of the present work was proved by the detailed investigations 

 of A. Fischer, 300 who showed that the vessels and tracheides of the 

 majority of our native deciduous and Coniferous trees always contain 

 considerable quantities of glucose in spring-time. The glucose content 

 begins to rise during the bleeding period, and increases still further 

 when the reserves of starch are mobilised. The concentration of sugar 

 in the vessels reaches a maximum about the beginning of May, and 

 falls off again gradually during the summer ; but a certain amount of 

 glucose is present even in winter. Fischer devised the following 

 experiment in order to demonstrate the ascent of glucose in the 

 transpiration-current. Leafless portions were selected from four-year- 

 old twigs of Acer dasycarpum in the latter half of May, when the entire 

 reserve of starch had been transformed into sugar; these were cut off 

 and placed with their lower ends in water. In such circumstances starch 

 appeared, immediately below the upper end, around the margin of the 

 pith and in the medullary rays, after 17 hours in a piece of twig 

 16 cm. in length, and in as little as 5 hours in a piece of half this 

 length. Similar results are obtained if the pieces of twig are inverted, 



