48 DEVELOPMENT OF THE ANTHER. 



The origin of the sacs in the way I have now described, 

 implies that pollen is homologous with the mesophyll of the leaf, 

 whereas their formation by involution of the blade would indi- 

 cate that substance to be a product of the epidermal tissue — an 

 idea certainly more in keeping with the origin of reproductive 

 cells in general. But as a matter of fact the pollen can be 

 plainly seen — an interstitial mass of cells lying within the walls 

 of the anther lobes, and I have not come across a single instance 

 in which I could detect an epidermal cell dividing into pollen 

 cells. 



Now, if these observations of mine are correct, it is hardly 

 likely that they will refer to an isolated mode of development. 

 There should be found analogous instances of the conversion of 

 parenchyma into reproductive tissue, and I believe that from 

 among the vascular cryptogams and the gymnosperms we shall 

 be able to cite cases of the production of spores in a manner 

 similar to that we have already explained. 



Speaking of the Sporangia in Selaginella and Lycopodiacea, 

 Sach says (page 415), " The youngest rudiments of Sporangia 

 which I could detect, but which I have frequently examined, 

 have the appearance of broad protuberances of the upper side 



of the young leaf Even in the youngest stages there 



can be recognised beneath the swelling of the epidermis a layer 

 of cells out of which, as the growth of the protuberance 

 advances, a spherical group of larger cells is formed, which 

 divides in all directions to form the mother cells of the spores." 



The sporangia of the Equisetacese bear a striking resemb- 

 lance to the young anther, and here again the mother cells of 

 the spores are formed out of the internal tissue. Referring to 

 the Cycadese, Sach says (page 440), " The mode of development 

 of the pollen sacs and pollen grains was till lately unknown —it 

 has only been quite recently observed. The pollen sacs are 

 formed on the underside of the stamens in the form of small 

 papillae, probably consisting from the first of several cells over 

 which the epidermis of the leaf is continuous. The inner tissue 

 is next differentiated as in the sporangia of Lycopodiacese, 

 Equisetacese, and Ophioglossese into an outer layer of smaller 

 cells enclosing a larger celled layer." 



