50 



VEGETABLE PHYSIOLOGY. 



[LESSON 17. 



FIG. 85. 



The foliage, consisting of leaves 

 ten or twelve feet in length, is always 

 produced at the summit ; and, as the 

 stem continues to grow, the leaves 

 make an impression on it which be- 

 comes permanent, and adds greatly to 

 its external beauty. 



For the reason above given, this 

 stem may be known at a glance ; but, 

 if a doubt exist, it will be immediately 

 removed by examining its internal 

 structure, as seen in transverse sec- 

 tion. 



FIG. 86. 



Tree Fern. 



Section of Tree fern, transverse. 



298. A transverse section of an 

 acrogenous stem (Fig. 86), shows a 

 circle of vascular tissue composed of 

 masses (a), of various forms and 

 sizes, situated near the circumfer- 

 ence; the centre (b) being either 

 hollow or formed of cellular tissue. On the outside of the vas- 

 cular circle, cells exist (c), covered by an epidermal layer (e), 

 often of hard and dense consistence, originally formed by the bases 

 of the leaves, which remain for a long time attached to the stem. 

 The vascular masses (a) are bordered by dark-colored woody fibres 

 (f) ; the pale-colored vessels, generally scalariform, which occupy 

 the centres of the vascular masses, are shown at g. The vascular 

 system is of greater density than the rest of the tissue, and is usually 

 distinguished by the dark color of the layer which surrounds the 

 paler vessels. 



