BKVELOPMeNI' Oi? STAMENS. 



171 



1 2' 3' ep cm 



479. Developmeut of the stamens. The following outline may 

 serve as an iutroduution to the stud}- of 

 the development of the stamens. At 

 first, the stamen exists as a mass of 

 homogeneous parench3-nia ; 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 dehcate pollen-sac.^ 



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 

 -S ij at right angles to the 



-tdv5fe^^^^s^v rp>.-^^^-n-r^ plane of the first division, 



thus producing four nuclei 

 forming a tetrahedron. 

 Cell-walls are n ext form ed, 

 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 muke up the layer Ibrming the pollen-sac are known, 

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

 has been termed the Tapetum. 



Fig. 131. Orchis inacnlata. 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 endothecium. The little mass 

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



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

 of this magnified; f, section through a young anther of Symphytum orientale; B, a 

 fourth of this niagniiied. The clotted lines in A and C show the part taken for exami- 

 nation. E, section of a young anther of Leucanthemnm vulgare ( W.-irraing.) 



