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_, × 50. _A_, surface view. _B-D_, optical sections. _s_, sepals, _p_, petals. _an._ stamens. _gy._ pistil. _E_, cross-section of the young anther, × 180. _sp._ spore mother cells. _F_, cross-section of full-grown anther. _sp._ pollen spores, × 50. _Fʹ_, four young pollen spores, × 300. _Fʺ_, pollen spores germinating upon the stigma, × 300. _pt._ pollen tube. _G_, young pistil in optical section, × 25. H, cross-section of a somewhat older one. _ov._ ovules. _I-L_, development of the ovule. _sp._ embryo sac (macrospore). _I-K_, × 150. _L_, × 50. _M_, embryo sac of a full-grown ovule, × 150. _Sy._ _Synergidæ_. _o_, egg cell. _n_, endosperm nucleus. _ant._ antipodal cells. _N-Q_, development of the embryo, × 150. _sus._ suspensor.]