138 MORPHOLOGY OF THE ANGIOSPERMS 



them by their large size, denser cytoplasm, and larger nuclei. These 

 cells, in elongate, often crescent-shaped or platelike rows, form the 

 archesporium (Fig. 67). The term archesporium, like so many other 

 terms used in Hower ontogeny, has had several meanings; it has been 

 applied not only to these rather readily recognized cell clusters but to 

 the central tissues at an earlier stage, from which arise not only the 

 sporogenous tissue and the tapetum but inner wall layers. The archespo- 

 rial cells — also called the primary sporogenous cells — enlarge radially 

 and divide periclinally, cutting off outer cells which form the tapetum. 

 The inner cells, after this division, form the primary sporogenous cells. 

 From the tissues outside tlie archesporium arise the epidermis and 

 wall layers. The terms applied to these cell layers as they develop — 

 primary parietal layers, parietal layers, primary tapetal layers — have 

 been so variously and inconsistently used that they are valueless. Doubt- 

 less, there is much variation in the behavior of the meristematic layers, 

 as in other meristems, and it is impossible here to ascribe more than 

 general origin and function to specific layers. The subepidermal 

 (fibrous) layer and one or more layers immediately below this are 

 clearly derived from the hypodermis of the young anther; deeper-lying 

 layers, even the layer that becomes the tapetum, apparently may have 

 different origins. 



The terms exothecium and endothecitim have been commonly applied 

 to outer and inner layers, respectively, of the mature anther-sac wall, 

 but are used as loosely as they were in reference to layers in the de- 

 veloping wall. Exothecium has been applied to the epidermis alone — 

 the common usage, to the epidermis plus the fibrous layer, and to the 

 outermost layer, regardless of its morphological nature. Endothecium 

 has been applied to the fibrous layer alone, to the wall layer or layers 

 below the fibrous layer, and to all of these together. The confusion in 

 use lies, in part, in the history of the terms, which are old and were at 

 first considered descriptive only. Early students of anther structure 

 overlooked the ephemeral layer below the fibrous layer, as well as the 

 occasional loss of the epidermis. The identity of layers in highly 

 specialized anthers can probably be determined only by ontogeny and 

 comparative studies of related taxa. Exothecium and endothecium have 

 only ecologic and topographic value; they are not of morphological 

 value. 



The anther wall consists, typically, of three layers: the epidermis; 

 the fibrous layer, immediately below the epidermis; and one or more 

 parietal or wall layers betweerj the fibrous layer and the tapetal cells. 

 But the anther wall may be reduced to only one or two layers (Fig. 

 59). Fibrous layer is a poor term, since it implies that the layer consists 

 of fiberlike cells. Its cells, though differing greatly from the surrounding 



