284 



PHOTOSYNTHETIC SYSTEM 



which envelops the entire circle of vascular bundles (or extends around 

 the fibrous cylinder) ; this arrangement is characteristic of cylindrical 

 or prismatic stems with a well-developed photosynthetic system, such 

 as those of Spartium juneeum, Genista braeteolata, Tunica Saxifraga and 

 spp. of Aparagus. 



Yet another type of structure, which is not uncommon among both 

 Monocotyledons and Dicotyledons, is characterised by the fact that the 

 tubular photosynthetic cells are arranged in more or less radial rows, 

 or at any rate in curved series, converging towards the parenchymatous 

 bundle-sheaths or the strands of conducting parenchyma ; here also a 

 certain proportion of the photosynthetic cells belong to the palisade-type. 



Fig. 114. 



T.S. through the leaf of Panicum turgidum. Photosynthetic system of the girdle-type. 



After Volkens. 



This girdle-structure, as it may be termed, is seen in its greatest perfection 

 in certain species of Cyperus (Fig. 113), in which each vascular bundle 

 of the leaf is surrounded by a layer of radially elongated photosynthetic 

 cells ; translocation probably goes on in a second green parenchymatous 

 sheath, which is situated immediately within the prosenchymatous 

 endodermis. This girdle-arrangement is likewise excellently illustrated 

 by many Grasses (e.g. Saccharum officinarum and Spartina cynosuroides), 

 more especially by species that inhabit desert-regions (Pennisctum 

 dichotomum, Cynodon Dactylon, Panicum turgidum [Fig. 114], Andro- 

 pogon Mrtus, A. foveolatus, Danthonia, Forshdii, according to Volkens). 

 Among the Grasses the sheath of conducting parenchyma, though in 

 the first place acting as the efferent tissue, also contains numerous 

 chloroplasts, which are moreover often large and very brightly coloured. 

 It is uncertain whether this green inner sheath meraly represents an 

 unimportant addition to the chlorophyll-apparatus of the plant, or 



