126 



MORPHOLOGY 



270, 271, 272). The outer cells are the primary wall cells, which by 

 subsequent divisions give rise to a sporangium wall of at least three 

 layers of cells. The inner cells are the primary sporogenous cells, which 

 by subsequent divisions give rise to a considerable mass of sporogenous 

 tissue (fig. 273). This method of sporangium formation, by which the 

 inner cells, following periclinal division of the superficial initials, give 

 rise to the sporogenous tissue, is called the eusporangiate method, and 

 plants exhibiting it are often spoken of as eusporangiates. All vascular 



273 



274 



FlGS. 271-274. Sporangium of Lycopodium: 271, 272, view of young sporangium 

 in different planes, showing primary wall layer, primary sporogenous tissue (shaded), 

 and the underlying subarchesporial pad; 273, section showing further development of 

 sporogenous tissue; 274, older sporangium, showing staLklike subarchesporial pad, 

 three wall layers, the innermost of which is the tapetum (shaded), and the rounded off 

 and separated spore mother cells. After BOWER. 



plants are eusporangiates except the modern ferns, whose peculiar 

 method of sporangium formation will be described later. 



The sporogenous tissue is invested by a special nutritive layer known 

 as the iapetum or tapetal layer (fig. 274). In Lycopodium the outer 

 portion of the tapetal layer is composed of the innermost wall layer, 

 and the inner portion of the sterile tissue contiguous to the sporogenous 

 tissue. The tapetum, therefore, is simply the layer of sterile cells 

 abutting against the sporogenous tissue, which have been transformed 

 into feeding celts. This function gives to the layer a very characteristic 

 appearance, making it quite distinct from the sterile tissue outside 



