MECHANISMS FOR CONVEYANCE TO AND FRO. 467 



to several centimetres. The materials which serve to promote the growth of the 

 branches of an Aspen are manufactured in the long-stalked, gi-een leaf-blades 

 of this plant. That they may reach the growing branch, they must pass down 

 the long leaf-stalk and travel along a road many thousand times exceeding 

 the length of those cells in which they were formed. Let us glance at a palm, 

 whose few large leaves, forming a plume, sway about at the summit of a slender 

 stem. In order to reach the growing roots, the constructive materials formed 

 in the green leaves have to travel over a road 20 or 30 metres long. The dis- 

 tance is still greater over which the sap prepared in the foliage of tropical vines 

 is conveyed in order to reach the roots, where it serves as food to pai-asitic 

 raflBlesias growing thereon. It is naturally to be expected that in such instances 

 the routes followed by the travelling materials, and also the starting and end 

 stations, should exhibit characteristic features. What has been ascertained on this 

 point may be here briefly set forth. 



The green tissue which is developed in by far the lai-ger number of cases in the 

 cortex of green stem-structures, in foliage-leaves, young fruits, &c., more rarely in 

 floral leaves and roots, must be regarded as the first or departure station. In the 

 green multicellular thallophytes and in mosses, the chlorophyll-containing cells also 

 form the channels of removal for the materials which have been formed in the cells, 

 and these are always extended lengthwise in accordance with the direction of the 

 stream. In the leaves of mosses very frequently cell-rows and cell-bands ai'ise 

 which converge towards the base of the leaf, and in the vicinity of these points the 

 cells are most elongated according to the direction of the cui'rent. The conducting 

 cells in the stem are also much elongated in the direction of the current. But here 

 no definite line can be drawn between the forms of the cells at the departure 

 station, in the channel, and at the termination of the current. 



It is difiierent in those plants whose leaves and stem are traversed by vascular 

 bundles. These cells devoid of chlorophyll, and peculiar tubes belonging to the 

 bundles, take up the materials proceeding from the green tissues to conduct them 

 to the places of consumption. Division of labour has been so far carried out in all 

 these cases that a portion of the cells undertakes the decomposition of carbonic acid 

 and the formation of the first organic compounds, and another the conveying away 

 of these first products; but obviously this does not preclude the possibility that 

 manifold changes may still take place during the transit. In such a division of 

 labour it is important that the organic compounds which have been formed in the 

 superficial green cells, under the influence of light, should be removed as quickly as 

 possible from the places where they are produced, so that the important process of 

 the decomposition of carbonic acid should suffer no kind of interruption. It is 

 on account of this rapid removal by the shortest path that the green cells are 

 elongated in the direction in which they transport their products, and that the 

 neighbouring green cells are separated as much as possible from one another. 

 However they may be arranged in other respects, the indicated direction and 

 isolation ai-e always observed by them under all circumstances. 



