DEVELOPMENT OF STAMENS. 



171 



479. Development of the stamens. The following outline may 

 serve as an introduction to the stud}- of 



the development of the stamens. At 

 first, the stamen exists as a mass of 

 homogeneous parenchyma ; later, a del- 

 icate fascicle, continuous with one in the 

 filament, becomes differentiated in one 

 part of the stamen, the connective. Four 

 longitudinal ridges appear on the an- 

 ther, which coincide with four lines of 

 large cells within. These cells give rise 

 to the mother-cells of the pollen and to 

 the very delicate pollen-sac. 1 



480. The mother-cells of the pollen 

 have at first thin walls, but later these 

 become irregularly thickened. In a 

 large number of cases many mono- 

 cotyledons, and most if not all dicoty- 

 ledons the nucleus of a mother-cell divides into two nuclei, 



which themselves divide 

 D at right angles to the 



plane of the first division, 

 thus producing four nuclei 

 forming a tetrahedron. 

 Cell-walls are next formed, 

 and four cells are pro- 

 duced, which are called 

 the tetrad. After the 

 mother-cells of the pollen 

 have been changed into 

 tetrads, the mass of pro- 

 toplasm in each of the 

 cells of a tetrad becomes 

 covered, as Strasburger 

 has shown, with a new 



1 The cells which make up the layer forming the pollen-sac are known, 

 collectively, as the Archcsporium. The epithelium which lines the pollen-sac 

 has been termed the Tapctum. 



FIG. 131. Orchis maculata. A pollen-mass in process of enlargement, with the anther- 

 wall on the outside: ep, epidermis; 1, layer of cells under the epidermis remaining un- 

 divided ; 2' and 3', layers arising from division ; 3', the endnthcriiim. The little mass 

 cm, formed by the mother-cells, is surrounded by a thickened wall. 3 f. (Gnignard ) 



FIG. 132. A, transverse section of a young anther of Mentha aquatica ; B, a fourth 

 of this magnified; C, section through a young anther of Symphytum orientale; D, a. 

 fourth of this magnified The dotted lines in A and C show the part taken for exami- 

 nation. E, section of a youiig anther of I^eucanthemum vulgare. (Warming.) 



