FORMATION OF THE MICROSPORES 221 



Stage of the x generation just as the fertilized egg is the one-celled 

 stage of the 2x generation. Prothallium, gametophyte, x genera- 

 tion are different names for the same thing. So, too, fern plant, 

 sporophyte, 2x generation are names referring to the same thing. 

 It will be observed that in Osmunda cinnamomea the value of x 

 is twenty-two. 



Spore-formation in Spermatophytes. — Let us now see what 

 are the visible processes in the formation of the spores of Sperma- 

 tophytes, or so-called flowering plants. In these the spores are 

 of two kinds, the microspores or pollen grains, and the mega- 

 spores, which are usually the same thing as the embryo-sac cells. 



Formation of the Microspores. — When the stamens first 

 appear as minute outgrowths from the receptacle, and for some 

 time thereafter, they consist solely of meristematic cells. These 

 proceed to construct an anther which early gives indications of 

 a four-lobed outline. The anther itself is at first but a mass of 

 meristematic cells, but soon the outer layer becomes differen- 

 tiated into an epidermis. Cross sections of such anthers at 

 different stages in their early development show in each lobe 

 a hypodermal cell (Fig. 127, A, a), or sometimes group of cells, 

 of larger size, richer protoplasmic content, and with larger nuclei 

 than the others. A longitudinal section shows these to be really 

 a line, or sometimes plate of cells (Fig. 127, F, a) extending 

 nearly the length of the anther. This line of cells is called the 

 archesporium. 



The archesporium divides throughout its length by periclinal 

 walls, producing an outer primary parietal, B, ppr, and an inner 

 primary sporogenous layer, B, ps. By anticlinal divisions the 

 parietal layer is extended part way around the sporogenous cells, 

 and then by periclinal divisions the former gives rise to an outer 

 and an inner layer, C, and i. By periclinal divisions the outer 

 layer now produces two layers, D and E, 0, which constitute a 

 part of the sporangium wall, while the inner layer proceeds to 

 function as tapetum or group of nurse cells, D and E, t, destined 

 to take active part in the nutrition of the spores, as we saw to be 

 the case in Aneimia. The tapetum is extended around the 



