350 LECTURE XV. 



causes the cells of the pith to be shorter and broader than 

 they otherwise would be. 



A good illustration of the effect of the transverse tension 

 which exists in growing stems independently of a secondary 

 growth in thickness, is afforded by the development of tyloses. 

 It had long been observed by histologists that the cavities of 

 the vessels of the wood of the stems and roots of various 

 plants are filled with delicate parenchymatous tissue, but the 

 mode of origin of this tissue was not clearly understood until 

 Reess made his researches upon the subject. It is formed in 

 this way. When a vessel having large bordered pits in its 

 wall abuts upon a parenchymatous cell, the delicate mem- 

 brane which separates the lumen of the vessel from the cavity 

 of the cell gradually grows into the lumen of the vessel so as 

 to form a considerable protuberance (Fig. 40), the tylose, 



FlG. 40 (after Weiss). Portion of a vessel (a) of Vitis vinifera with adjoining 

 wood-cells (c), one of which has grown into the lumen of the vessel to form a 

 tylose (). 



which may even undergo cell-division. This takes place at 

 several points in the wall of the vessel, and thus its lumen 

 becomes filled with parenchymatous tissue. The physiolo- 

 gical explanation of the phenomenon is this. The turgid 

 parenchymatous cells are compressed longitudinally by the 

 resistance offered by the fibrous and vascular tissues to 

 their elongation : the effect of this is that the parenchymatous 

 cells tend to expand laterally, and they press with great force 

 against the lateral walls of the fibres and vessels. The pres- 



