472 MECHANISMS FOR CONVEYANCE TO AND FRO. 



bundles, which run almost parallel from the base to the apex of the leaf. Each of 

 the bridge-like green cells is connected at either end with a vascular bundle, and 

 delivers the materials produced to the conducting portions of these vascular 

 bundles on both sides, i.e. to the vascular bundle sheaths. In other liliaceous 

 plants, especially in the leaves and green stems of species of onion, the green cells 

 are palisade-shaped, and their longer diameter is perpendicular to the surface of the 

 part to which they belong. Here we have only a one-sided connection with the 

 conducting cells of the vascular bundle, but the junction is again a direct one. 

 Finally, the peculiar connection of laticiferous tubes with the palisade-cells in the 

 leaves of species of spurge must be considered. 



Although the laticiferous tubes appear to be very much branched wherever they 

 occur in plants, the formation of branching tubes is nowhere else so abundant as 

 in the vicinity of the palisade-cells. Many of the twigs directly adjoin these cells. 

 It also happens that single terminations of the laticiferous tubes impinge upon the 

 lower ends of several palisade-cells, which are inclined towards one another (fig. 

 25a, s), and that single laticiferous ramules push their way in between these 

 cells. In all these examples the materials manufactured in the green tissue 

 are taken up without further intervention by the ultimate terminations of the 

 conducting laticiferous tubes. 



Of the second group, which is characterized by the fact that the junction is 

 brought about by specially intercalated cells, the first instance to be considered is 

 that often observed in the leaves of grass-like plants, where the intermediate cells, 

 which are also called conducting cells, are more or less extended transversely, and 

 unbranched. The green cells lying under the epidermis are palisade-shaped, and 

 at right angles to the leaf -surf ace ; the longer diameters of the cells lying below 

 these, which are much poorer in chlorophyll-corpuscles, are, on the other hand, 

 placed obliquely to the leaf -surface, or even parallel to it, and apparently are 

 directed towards the large cells of the bundle sheaths in the middle of the leaf. 

 These cells, poor in chlorophyll, therefore connect the palisade-cells with the con- 

 ducting cells of the bundle sheath, and serve as agents in the removal of the 

 substances. But the commonest cases are those in which the conducting cells 

 are much branched. The foliage -leaves which possess these branched cells are 

 differently constructed on the upper and under sides of the leaf. Under the 

 epidermis of the upper side is seen the palisade-tissue, consisting of cylindrical or 

 prismatic cells, whose long axis is directed perpendicularly to the plane of the leaf 

 (see fig. 62^ and fig. 25a, r). Below these palisade-cells come the branched cells, 

 which are connected with one another by their arm -like processes, forming 

 a large-meshed tissue, the frequently-mentioned spongy parenchyma, interrupted 

 by wide interstices. The spongy parenchyma is connected with the palisade-tissue 

 by means of single processes bordering the lower, that is to say, the inner ends of 

 the palisade-cells; very often a single process is connected with the inner ends of 

 several palisade-cells, in which case these have a clustered arrangement. As with 

 the palisade-cells, the branched cells of the spongy parenchyma are connected with 



