142 MORPHOLOGY OF THE ANGIOSPERMS 



Najos, the simple-walled anther is enclosed by two sheathing structures. 

 Freeing of the pollen is by a forcing apart of the enclosing sheaths, the 

 outer one by growth of the receptacle and peduncle below the flower. 

 Whether the single wall layer of Nojas represents the epidermis, the 

 fibrous layer, or some sheathing organ is not known. 



The Tapetum. The primary sporogenous cells enlarge radially and 

 divide periclinally. The outer cells so formed constitute the tapetum, a 

 layer that encases the sporogenous cells and serves as a nutritive tissue 

 for the pollen mother cells and the microspores (Fig. 67). The layer is 

 typically uniseriate, rarely biseriate, but often irregular in outline and 

 thickness, especially on the inner side of the sporogenous tissue. Tapetal 

 cells may be formed also by transformation of outer sporogenous cells 

 and, occasionally, from isolated sporogenous cells. The tapetum is 

 described as derived, in some taxa, from the innermost layer of primary 

 wall cells; in other taxa, partly from this layer and partly from the 

 sporogenous cells. On the inner side of the sporogenous cell mass, it 

 may arise from parenchyma of the connective and here complete the 

 enclosure of the fertile cells. The tapetal cells enlarge rapidly, and 

 their cytoplasm becomes denser. The tapetum is commonly uninucleate 

 but may become bi- or multinucleate. The presence of many nuclei has 

 suggested amitotic division, but an appearance of amitosis is perhaps 

 given by incomplete mitosis. The mature tapetal cells may be uniformly 

 uni-, bi-, or multinucleate or may vary in number of nuclei. At the 

 time of spore formation and immediately afterward, the tapetal cells 

 break down, and their cytoplasm unites to form a tapetal plasmodiiim. 

 The cytoplasm is freed in different ways — by disintegration of the walls 

 or by extrusion through collapsing walls. Cells of the connective abutting 

 on the tapetum may also break down, but it is uncertain whether their 

 contents are added to the plasmodium. The tapetal plasmodium extends 

 among the spore tetrads or the free spores, isolating them singly or in 

 clusters. As the spores mature, the plasmodium nourishes them and 

 builds their walls. 



Two types of behavior of the tapetal cells in the development of the 

 pollen grains are distinguished: the amoeboid and the secretory. In the 

 amoeboid type, the protoplasts of the tapetal cells enlarge and intrude 

 among the spore mother cells, spores, and young pollen grains, before 

 merging as a periplasmodium; in the secretory type, the tapetal cells, 

 without collapsing, become rich in content and "secrete" a periplas- 

 modium. In both types, the periplasmodium ultimately surrounds the 

 maturing spores or pollen grains. 



Four types of amoeboid formation of the periplasmodium have been 

 distinguished, but these differ perhaps too little to be important. In the 

 Sagittaria type, the plasmodium develops rather late — about the time 



