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ened rings or with pits in the walls. The small, spirally-marked vessels are nearest the centre, and are the first to be formed in the young bundle. The epidermis of the leaves is composed of irregular cells with wavy outlines like those of the ferns. Breathing pores, of the same type as those in the ferns and monocotyledons, are found on both surfaces, but more abundant and more perfectly developed on the lower surface of the leaf. Owing to their small size they are not specially favorable for study. The epidermis is sparingly covered with unicellular hairs, some of which are curiously branched, being irregularly star-shaped. The walls of these cells are very thick, and have little protuberances upon the outer surface (Fig. 93, _E_). Cross-sections of the leaf may be made between pith as already directed; or, by folding the leaf carefully several times, the whole can be easily sectioned. The structure is essentially as in the adder-tongue, but the epidermal cells appear more irregular, and the fibro-vascular bundles are better developed. They are like those of the stem, but somewhat simpler. The xylem lies on the upper side. The ground tissue is composed, as in the leaves we have studied, of chlorophyll-bearing, loose cells, rather more compact upon the upper side. (In the majority of dicotyledons the upper surface of the leaves is nearly or quite destitute of breathing pores, and the cells of the ground tissue below the upper epidermis are closely packed, forming what is called the "palisade-parenchyma" of the leaf.) [Illustration: FIG. 95.--_A-D_, successive stages in the development of the flower of _Capsella_, x 50. _A_, surface view. _B-D_, optical sections. _s_, sepals, _p_, petals. _an._ stamens. _gy._ pistil. _E_, cross-section of the young anther, x 180. _sp._ spore mother cells. _F_, cross-section of full-grown anther. _sp._ pollen spores, x 50. _F'_, four young pollen spores, x 300. _F"_, pollen spores germinating upon the stigma, x 300. _pt._ pollen tube. _G_, young pistil in optical section, x 25. H, cross-section of a somewhat older one. _ov._ ovules. _I-L_, development of the ovule. _sp._ embryo sac (macrospore). _I-K_, x 150. _L_, x 50. _M_, embryo sac of a full-grown ovule, x 150. _Sy._ _Synergidae_. _o_, egg cell. _n_, endosperm nucleus. _ant._ antipodal cells. _N-Q_, development of the embryo, x 150. _sus._ suspensor.] The shepherd's-pur
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