SPEEMAPHTTES : PH^ENOGAMS. 135 



a dark-colored ground tissue. Growing from its surface are 

 numerous root hairs (Fig. 75, M ) . 



For examining the microscopic structure of the pine, fresh material is 

 for most purposes to be preferred, but alcoholic material will answer, and 

 as the alcohol hardens the resin, it is for that reason preferable. 



Cross- sections of the leaf, when sufficiently magnified, show that the 

 outer colorless border of the section is composed of two parts : the epi- 

 dermis of a single row of regular cells with very thick outer walls, and 

 irregular groups of cells lying below them. These latter have thick walls 

 appearing silvery and clearer than the epidermal cells. They vary a good 

 deal, in some leaves being reduced to a single row, in others forming very 

 conspicuous groups of some size. The green tissue of the leaf is much 

 more compact than in the fern we examined, and the cells are more nearly 

 round and the intercellular spaces smaller. The chloroplasts are numer- 

 ous and nearly round in shape. 



Scattered through the green tissue are several resin passages (r), each 

 surrounded by a circle of colorless, thick-walled cells, like those under the 

 epidermis. At intervals in the latter are openings breathing pores 

 (Fig. 76, /), below each of which is an intercellular space (i). They are 

 in structure like those of the ferns, but the walls of the guard cells are 

 much thickened like the other epidermal cells. 



Each leaf is traversed by two fibro-vascular bundles of entirely dif- 

 ferent structure from those of the ferns. Each is divided into two nearly 

 equal parts, the wood (x) lying toward the inner, flat side of the leaf, the 

 bast (T) toward the outer, convex side. This type of bundle, called 

 "collateral," is the common form found in the stems and leaves of seed 

 plants. The cells of the wood or xylem are rather larger than those of 

 the bast or phloem, and have thicker walls than any of the phloem cells, 

 except the outermost ones which are thick- walled fibres like those under 

 the epidermis. Lying between the bundles are comparatively large color- 

 less cells, and surrounding the whole central area is a single line of cells 

 that separates it sharply from the surrounding green tissue. 



In longitudinal sections, the cells, except of the mesophyll (green tissue) 

 are much elongated. The mesophyll cells, however, are short and the 

 intercellular spaces much more evident than in the cross-section. The 

 colorless cells have frequently rounded depressions or pits upon their 

 walls, and in the fibro-vascular bundle the difference between the two 

 portions becomes more obvious. The wood is distinguished by the presence 

 of vessels with close, spiral or ring-shaped thickenings, while in the phloem 

 are found sieve tubes, not unlike those in the ferns. 



