SPERMATOPHYTES 



257 



which is the more or less elongated, stalklike region (figs. 572-574). 

 It should be noted that such a term as anther is one of convenience 

 rather than of morphological exactness, for it is made up of a complex 

 of sporangia and sporophyll. 



Microsporangia. The sporangia develop as in gymnosperms, being 

 of the eusporangiate type. A transverse section of a very young anther 

 shows a mass of homogeneous tissue invested by the epidermis. The 

 layer just beneath the epidermis (hypodermal layer) is potentially 

 sporogenous; but usually it becomes actually sporogenous in four 

 regions, which in transverse section show a variable number of cells 

 (one to several). Of course 

 these regions of initial cells 

 are really four longitudinal, 

 hypodermal bands of varying 

 width. Each one of these 

 bands of initials divides peri- 

 clinally, forming two layers 

 of cells (fig. 575). The outer 

 layer (just beneath the epi- 

 dermis) is the primary wall 



layer; the inner one is the 572 573 11! 574 



primary sporogenous layer. 

 The primary wall layer di- , FlGS ' 57*-574. - Stamens of angiosperms, 

 J , showing anther and niament: 572, ordinary type, 



Vides further, forming several with longitudinal dehiscence; 573, Solanum, with 

 (usually three to five) wall dehiscence by terminal slit or pore; 574, Vuc- 



layers (fig. 576). The outer- ~' with tubul A a f r P lon g ations of P ollen sacs 



' for dehiscence. After KEENER. 



most wall layer is usually 



much modified, the cells becoming large and conspicuously banded, 

 forming the so-called endolhecium (fig. 580), a layer that assists in the 

 dehiscence of the sporangium. The innermost wall layer usually 

 becomes transformed into a portion of the tapetum, the nutritive layer 

 of the sporogenous tissue (figs. 576, 577). The intermediate layers are 

 the middle layers, and usually become more or less flattened and dis- 

 organized through the activity of the tapetum. A section through a 

 completed sporangium wall, therefore, reveals the epidermis, the 

 endothecium, one or more middle layers, and the tapetum (fig. 577). 

 The cells of the primary sporogenous layer usually divide two or three 

 times (sometimes oftener, and sometimes not at all), forming the spore 

 mother cells (fig. 577). In the two successive divisions of the mother 



C. B. & C. BOTANY I 



