178 



BOTANY. 



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 



1) 



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, ay. 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. Synergidse. o, egg cell, n, endosperm 

 nucleus, ant. antipodal cells. JV-Q, development of the embryo, x 150. 

 sus. suspensor. 



quite destitute of breathing pores, and the cells of the ground tissue below 

 the upper epidermis are closely packed, forming what is called the "pali- 

 sade-parenchyma" of the leaf.) 



The shepherd' s-purse is an admirable plant for the study of the develop- 

 ment of the flower which is much the same in other angiosperms. To 

 study this, it is only necessary to teaze out, in a drop of water, the tip of 



