THIRD TYPE OF CONSTRUCTION 



285 



whether there exists some as yet undiscovered division of labour 

 between the chloroplasts in the sheath and those in the girdle-cells. 



Third System. The third and most effective type of photosynthetic 

 system is characterised by the fact that the synthetic products are not 

 transferred directly from the photosynthetic elements to the efferent 

 channels, but first pass through a special intermediary tissue {Zuleitungs- 

 gewebe). This scheme of construction comprises two sub-types, which 

 differ from one another in one important feature ; in one case, namely, 

 the efferent vascular strands all follow more or less parallel courses, 

 whereas in the other they form a complicated reticulum. The inter- 

 mediary tissue is accordingly composed, in the former case of transversely 

 elongated cells, and in the latter of elements with several radiating 



Fig. 115. 

 T.S. through a bract of Cyperus alternifolius. x300. 



branches. The actual photosynthetic tissue again consists of palisade- 

 cells. The first-named type of structure prevails in the leaves of most 

 Grasses and Carices, and in many species of Cyperus ; further, in certain 

 Liliaceae, among Cycads and in Taxus haccata. The other type is 

 the normal arrangement among Dicotyledons, while it likewise occurs 

 in the majority of Ferns. 



The anatomy of the bracts of Cyperus alternifolius (Fig. 115) will 

 serve to illustrate the type of photosynthetic system that is character- 

 istic of the Glumiflorae. Here the web of each I-girder consists of 

 a vascular bundle enclosed in a sheath of large colourless cells. The 

 photosynthetic tissue takes the form of cell-plates situated above and 

 below the air-passages that alternate with the girders. On the 

 adaxial side of the leaf it consists of palisade-cells ; in transverse 

 sections these cells appear to be in uninterrupted contact with one 

 another, but a radial section shows that they are separated from one 

 another by intercellular spaces of various sizes. At the margins of each 



