MORPHOLOGY 



cells to form the tetradsNpf 

 spores (fig. 578), the reductioiji 

 of chromosomes occurs. UsuA 

 ally the tetrads within the 

 mother cells are of the tetra- 

 hedral type, but in some cases 

 the arrangement is different, 

 the four spores being in a 

 linear series. The mature 

 microspores (pollen grains) 

 usually round off and sepa- 

 rate, forming a powdery mass 

 (fig. 580). The spore walls 

 are two-layered, the outer 

 layer (exine) being thicker 

 and more brittle and often 

 variously sculptured, the inner 

 layer (intine) being delicate 

 and very elastic. In the exine 

 there are always one or more 

 thin spots where the pollen 

 tubes emerge, most monocoty- 

 ledons having one such spot, 

 and dicotyledons having two 

 to many. In the pollen 

 grains of Ranunculus (butter- 

 cup), for example, fifteen to 



FIGS. 575-578. -Development of the micro- thirt y thin S P tS ^ be ^ 



sporangium of Silphium: 575, transverse section served. 



of a small portion of a very young anther, show- T <, ome ff roUDS o f anffio- 

 ing the hypodermal initial cells (shaded and 



with large nuclei), which are beginning the peri- sperms the Spores of a tetrad 



clinal division into primary wall cells (outer) d t separate a condition 



and primary sporogenous cells; 576, later stage, \ 



the primary wall cells having formed three once described as a Compound 

 layers, the innermost (shaded) being the tapetum T certain CaSCS Still 



(nutritive layer), which is continuous about the & ram ' 



sporogenous cells; 577, later stage, showing en- larger groups of Spores cling 



dothecium (layer beneath epidermis) forming, to{rpt u er anf 1 t Uj s fendencv 

 middle layer of flattened cells, tapetum conspic- er ' ' 



ucus and its cells binucleate, and sporogenous reaches its extreme expression 



tissue (after one division of primary sporogenous j fa , fe ^ orchids and 



cells) m the mother cell stage; 578, a mother 



cell containing a tetrad. After MERRELL. milkweeds, where all the 



