THE LEAF. 3 



of subspherical cells) and this surrounded by wood a layer of fibro- 

 vascular tissue which in turn is surrounded by bark. Such a stem 

 increases in thickness by the growth of new tissue between the wood and 

 bark, adding each year a new sheath ring as seen in cross-section to 

 the outside of the wood (whence the name) and a new lining to the bark. 

 All- the trees of the colder climates belong to this class. 



15. There exists in the wood of exogenous stems, besides the fiber 

 described above (8-11), medullary rays, which make the silver-grain in. 

 the language of wood workers. They are narrow plates radiating from 

 pith to bark, and are composed of short flattened cells. They are shown 

 in Fig. 2, m, as cut across and highly magnified, and in Fig. 3 as seen 

 from all sides. The trans- 

 verse section, Q (Fig. 3) 



shows them as seen from 

 above, resembling lines radi- 

 ating out from the center. 

 The radial section, Sp, shows 

 their width, the transverse, 

 Q, their thickness and 

 length, and the tangen- 

 tial, Sc, as cut across. Their 

 great range of variation in 

 size and appearance in dif- 

 ferent timbers can be beauti- 

 fully studied in the wood 

 sections, which are designed 

 to be cut on those planes. 



16. The Endogenous Stem. (L. endo, within., and genus, origin) is 

 characterized by having the wood disposed in threads or bundles (called 

 fibro-vascular bundles) throughout the pithlike substance of the interior. 

 It increases in thickness by the growth of new bundles within the -stem 

 (whence its name), and it has no real bark. The class is represented by 

 the Palms and similar trees of tropical countries, and in colder climates 

 by many herbaceous plants. The Indian Corn is a familiar example in 

 the North and the Palmetto in the South. 



FIG. 3. 



THE LEAF. 



17. This is a thin, expanded organ for presenting a large amount of 

 surface to the action of sun-light and air, and this for the performance 

 of one of the most important workings of Nature. The function referred 

 to is known as assimilation, or the conversion of mineral or inorganic 

 matter into living or organic matter, and upon it directly or indirectly 

 depend all forms of life. Nowhere else in the whole realm of Nature is 

 this work performed than in plants, and there by the agency of its leaves, 

 to which the rest of the plant each part in its way is accessory. 



18. How the sap is brought up through the substance of the stem to 



Fig. 3. Structure of the Oak at two years of age, showing, m (upper side), pith consisting 

 of, m', spongy portion and, m (under side), medullary sheath ; ?t, wood of two years growth, jj 

 being the dividing line; c, cambium layer; r, bark and, 1-7, medullary rays, (^represents the 

 transverse section, Sp the radial and Sc the tangential sections. (From Hough's Elements 

 of Forestry.) 





