534 PHYSIOLOGY. 



chyma, in which is plunged a central cylinder. The epidermis 

 will be spoken of hereafter. 



The Cortical Parenchyma is wholly cellular, and may be gene- 

 rally subdivided into two zones, an outer and an inner. The cells 

 of the outer zone increase from within outwards (centrif ugally) , 

 decrease in size towards the outside, and are so closely packed as 

 to leave no intercellular spaces. The cells of the inner zone of 

 the cortical parenchyma increase from without inwards (centri- 

 petally), and decrease in size from without inwards. They are 

 disposed in radiating series or in concentric zones, and have inter- 

 cellular spaces. 



The Protecting Sheath. This, the game protectrice of the 

 French, the Schutzscheide of the Germans, was first pointed out by 

 Caspary. It is a special layer of cells, forming the innermost 

 layer of the cortical parenchyma and separating it from the central 

 cylinder. The cells of the protecting sheath are marked on their 

 sides by transverse folds or ridges, the ridges of one cell fitting 

 into corresponding furrows of its neighbour, so that the cells are 

 " dove-tailed " into one another. In longitudinal section these 

 undulations of the cell-walls give an appearance as of the rounds 

 of a ladder ; in transverse section the appearance is given of very 

 small oval dark spots in the middle of the lateral boundaries of the 

 cell. 



The Central Cylinder (Pericanibmm) is originally a mass of 

 pleroma or growing cellular tissue : the outermost layer of this 

 develops into a special layer of pericambium, which is thus in im- 

 mediate contact with the protecting sheath on the outer side, its 

 cells being so arranged as to alternate with those of the sheath. 

 Thus one cell of the pericambium comes between two of the sheath, 

 and so on. Hence the layer is readily recognizable under the 

 microscope. The central mass of plerorne forms the vessels which 

 are developed centripetally and alternately with bundles of bast 

 or liber-cells, also developed centripetally. The untransformed 

 cellular tissue surrounding the vessels and bast-cells is called the 

 connecting tissue. Hoots of this character, then, differ especially 

 from the stem in having their bast or liber-cells alternating in 

 position with the vascular bundles proper, not placed external to 

 them. 



Roots of Vascular Cryptogamia. The structure of the roots of the 

 higher Cryptogams is identical in the main with that just described ; 

 when once the primary structures are completed no further change takes 

 place. The new rootlets originate from the inner layers of the cortical 

 parenchyma. The roots of Equisetaceae have no pericambial layer. 



Roots of Monocotyledons. The general structure is the same as that of 

 the Vascular Cryptogams, the rootlets originating from the pericambial 



