SECOND TYPE OF CONSTRUCTION 



283 



sheaths of the minor vascular bundles, and partly by the larger plates 

 and strands of conducting parenchyma. 



Pinus illustrates a type which is in some respects even simpler 

 than that which has just been described, but which is, on the other 

 hand, complicated by the introduction of arm-palisade-cells (Fig. 110 a). 

 Full consideration has already been given, in the preceding section of 

 this chapter, to the structure and arrangement of the flanged tabular 

 cells of this genus ; it may be noted in addition, with reference to the 

 principle of expeditious translocation, that the photosynthetic tissue of 

 Pinus is made up of sharply defined cell-plates, each composed of a single 

 layer of cells which are in contact 

 with one another only at a few 

 points, being otherwise separated by 

 air-spaces. In these circumstances, 

 it is obvious that the synthetic 

 material manufactured in each indi- 

 vidual cell-plate must be transferred 

 directly to the parenchymatous sheath 

 and conducting parenchyma of the 

 central cylinder. A similar trans- 

 versely laminated photosynthetic 

 system is found also in certain 

 other Conifers, such as Thuja plicata, 

 Cryptomeria clegans (Fig. 116 c), 

 species of Abies, etc. 



A third modification of the 

 second system, which is especially 

 common among Monocotyledons, shows a further advance in the develop- 

 ment of typical palisade-layers, which are supported by an efferent 

 tissue arranged without any definite relation to the vascular bundles ; 

 this condition is exemplified by the leaves and stems of the genera 

 Allium, Ornithogalum, Asphodelus, etc. Here, again, the photosynthetic 

 tissue is abundantly provided with transverse air-spaces, which prevent 

 the outgoing stream of material from travelling for any appreciable 

 distance within the palisade-tissue itself. The synthetic products 

 cannot therefore, by any means, pass from the site of their formation 

 in a direct line across the palisade-cells to the base of the leaf; they 

 are on the contrary forced, in consequence of the arrangement of the 

 mesophyll, to pursue a centripetal course, and thus, by following the 

 long axis of the palisade-cells, to ]^ave the photosynthetic elements as 

 quickly as possible. 



In the case cf a very similar and closely related type of structure 

 palisade-tissue is associated with a common parenchymatous sheath, 



Fig. 113. 



T.S. through one of the smaller vascular 

 strands of the foliage-leaf of Papyrus cicuta. 

 The bundle is enclosed within three concentric 

 sheathing layers, viz. an internal and an ex- 

 ternal sheath of green cells, separated from one 

 another by the prosenchymatous endodermis. 

 The external green sheath, which consists 

 of radially elongated cells, represents the 

 specialised photosynthetic tissue. 



