468 MECHANISMS FOR CONVEYAN'CE TO AND FRO. 



The isolation is brought about by the elongated cells, which lie parallel side 

 by side, assuming a cylindrical form in consequence of which they mei-ely touch one 

 another, leaving large air-spaces between. An exchange of materials between these 

 cylindrical cells, i.e. a passage of materials transversely across their elongated sides 

 is wholly prevented, and the transport of the materials is effected only in that 

 direction in which the cylindrical cell in question is elongated. The organs which 

 convey the materials away from the green cells lie within the strands which form 

 the veining of the leaf, which traverse the leaf-stalk and stem as thick bundles, and 

 when densely aggregated, form the chief part of the trunks of woody plants. But 

 it would be erroneous to suppose that these strands {i.e. the vascular bundles) are 

 composed exclusively of structures for conveying away plastic materials. Adjoin- 

 ing these, and connected with them, are regularly found woody cells, tubes, and 

 other vessels, which conduct the mineral food-substances absorbed by the roots, and 

 the water in which these are dissolved, upwards to the transpiring tissues. Finally, 

 elastic thread-like bast-cells are always added to these structures, which serve for 

 the two kinds of transport, by which means the whole is given the necessary firm- 

 ness and elasticity. In these strands, therefore, which are called vascular bundles, 

 the most vai'ied structures with widely -differing functions are found crowded 

 together in a small space, and it happens that the cells and vessels which serve as 

 the passage for the current of organic materials formed in the green tissues, only 

 occupy a very moderate share of the space. 



Four kinds of mechanisms for carrying on the work of removal have been 

 discovered. First of all, there are groups of parenchymatous cells which adjoin the 

 other elements of the vascular bundle, especially the water-conducting woody cells 

 and vessels which they usually surround, forming an actual mantle round them, 

 termed the vascular bundle sheath. These vascular bundle sheaths are particularly 

 well developed in the foliage-leaves, and form there an important constituent of 

 the leaf-ribs and veins traversing the green tissue (see fig. 126 -). In the finest 

 and most delicate veinlets, representing the ultimate terminations of the vascular 

 bundles, the few water-conducting cells, stiffened by spiral thickenings, are 

 surrounded by parenchymatous cells. These are continued on beyond the vascular 

 bundle, and frequently the finest veinlets are formed to such a lai-ge extent of these 

 parenchymatous cells that they have been distinguished as a particular form of 

 tissue by the name of nerve-parenchyma. 



Next to the vascular bundle sheaths, medullary rays are to be regarded as 

 organs for convej'ing the formed materials fi'om the green leaves. These consist 

 also of parenchymatous cells with lignified walls which are elongated at right 

 angles to the axis of the stem-structure to which they belong. They form layers of 

 tissue which are situated between the vascular bundles, and connect the pith in the 

 centre of the stem with the cortex. Besides these medullary rays, which are known 

 as primary, quite similar layers are formed of parenchymatous cells in the body of 

 the vascular bundles, which, however, are in no way connected with the pith in the 

 centre of the stem, and which are known as secondary medullary rays. On cutting 



