12 



MORPHOLOGY OF SPERMATOPHYTES 



monly spoken of as the " primary tapetum," but since it does 

 not always give rise to the tapetum it would seem better to refer 

 to it as primary wall cells of the sporangium. By continued 



FIG. 9. Microsporophylls of Cycads : A, Cycas circinalis ; a, entire sporophyll, showing 

 sporangia on under surface ; 6, four groups of sporangia ; c, same as ft, after the 

 pollen has been shed. B, Zamia in tegri folia ; a, entire sporophyll ; b and c, clusters 

 of sporangia. Ab and Ac after BLUME, the other illustrations after KICHARD. 



periclinal divisions a wall consisting of three to six layers 

 (Stangeria) of cells is organized beneath the epidermis. 



The larger cells of the inner daughter plate of the first divi- 

 sion of the archesporial cells are the primary sporogenous cells 

 (Fig. 10), which by division organize a large sporogenous mass. 

 Lang states that in 8tangeria, after a considerable mass of 

 sporogenous cells is formed, the tapetum is organized by the 

 sporogenous tissue, either by cutting off peripheral tabular 

 cells or by direct conversion of the peripheral cells. If this 

 be true, the functional tapetum is derived from sporogenous 

 rather than wall tissue. The tapetal cells are distinguished in 

 the usual way by their size and deeply staining contents, and in 

 a single layer completely invest the sporogenous mass. The 

 tapetum persists as a distinct layer, but its walls disappear, and 

 its nuclei often fragment. In its development two of the adja- 

 cent wall layers are disorganized and appear as a thin band 

 about it, contributing of their material to the tapetum, and in 

 this sense functioning as a tapetum. 



With the complete organization of the tapetum division in 

 the sporogenous tissue ceases, and the cells become mother cells. 

 Tapetal work, which seems to be merely the contribution of 



