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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 
FIGS. 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 tapetum 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 cells. This function gives to the layer a very characteristic 
appearance, making it quite distinct from the sterile tissue outside 
