MECHANISMS FOR CONVKYAXCE TO AND FUO. 473 



tlio parenchyma sheaths of the veins. Thus the branched cells of the spoii<fy 

 parenchyma become agents in the transportation of the materials; with one branch 

 they take up the organic substances manufactured in the palisade-cells, and with 

 jmother they deliver these materials up to the bundle sheath for further translation 

 to the places of consumption or storage. 



That the cells of the spongy parenchj-ma serve not only for conduction, but 

 have to perform several other functions, does not need to be confii-med in detail. 

 It is enough to point out that they contain chlorophyll-corpuscles, and therefore 

 are capable of decomposing carbonic acid and of forming carbohydrates, although 

 to a much less extent than the palisade-cells, which are so richly supplied with 

 chlorophyll, ^loreover, the excretion of aqueous vapour occurs in the spongy 

 pai-enchyma whose lacunsB and passages communicate with the outer world by the 

 stomata, and where also a vigorous inflow and outpouring of other gases takes 

 place. Then the part which the conducting structures play in the metabolism 

 of the materials must not be overlooked. All these structures contain active 

 living protoplasm, in all there is a protoplasmic cell -body, although very 

 often it is only in the form of a delicate parietal layer, and in all, under the 

 iufluenQe of this living pi-otoplasm, we have not merely a movement, but also an 

 inexhaustible and infinite changing of the materials, corresponding to the indi- 

 viduality of the species and to the requirements of the time being. These structures 

 must then be regarded not onlj"- as simple channels for the fresh carbohydrates 

 produced in the green cells, but also as regions for transformations, where the first 

 organic compounds manufactured in the green cells are prepared for ultimate con- 

 sumption at the end of the journey. It is precisely in this respect that they differ 

 essentially from that conducting apparatus, whose task is to transmit water and 

 mineral salts to the green tissues, and which, as already repeatedly remarked, is 

 present in the same bundle as the cells and vessels which take away the organic 

 materials. When once the water-conducting tubes and cells have attained their full 

 dimensions, they no longer contain protoplasm, and no transformation of the trans- 

 mitted raw food-sap is carried on in them; the water, with the mineral food-salts 

 dissolved in it, is carried through them unaltered to the transpiring cells. To 

 emploj' the simile, often used before, of the arrangements of a well-conducted house- 

 hold, the woody cells and vessels of a vascular bundle may be compared to an 

 apparatus for delivering water and salts into the kitchen, so to speak; the green 

 tissue forms the kitchen in which the raw materials are worked up and so prepared 

 that they can be brought back by the removing cells to the places where they are 

 required and consumed. 



That these two fundamentally different kinds of conducting apparatus arc so 

 universally found united together in one and the same bundle is explained by the 

 fact that the places which form the goal for one are at any rate to some extent 

 the starting-point of the other; besides, of course, this combination economizes the 

 building materials. All conducting apparatuses must be strengthened and pro- 

 tected in their position, ami therefore it is beneficial and saves building materials if 



