2s8 



MORPHOLOGY 



Figs. 575-57S- — Development of the micro- 

 sporangium of Silphium: 575, transverse section 

 of a small portion of a very young anther, show- 

 ing the hypodermal initial cells (shaded and 

 with large nuclei), which are beginning the peri- 

 clinal division into primary wall cells (outer) 

 and primary sporogenous cells; 576, later stage, 

 the primary' wall cells having formed three 

 layers, the innermost (shaded) being the tapetum 

 (nutritive layer), which is continuous about the 

 sporogenous cells; 577, later stage, showing en- 

 dothecium (layer beneath epidermis) forming, 

 middle layer of flattened cells, tapetum conspic- 

 uous and its cells binucleate, and sp)Orogenous 

 tissue (after one division of primary sporogenous 

 cells) in the mother cell stage; 5 78, a mother 

 cell containing a tetrad. — After Meerell. 



cells to form the tetrads of 

 spores (fig. 578), the reduction 

 of chromosomes occurs. Usu- 

 ally the tetrads within the 

 mother cells are of the tetra- 

 hedral t^-pe, 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. 5S0). The spore walls 

 are two-layered, the outer 

 layer (exinc) being thicker 

 and more brittle and often 

 variously sculptured, the inner 

 layer (inline) being delicate 

 and very elastic. In the e.xinc 

 there are always one or more 

 thin sputs where the pollen 

 tubes emerge, most monocoty- 

 ledons having one such spot, 

 and dicotyledons having two 

 to manv. In the pollen 

 grains of Ranunnihis (butter- 

 cup), for example, fifteen to 

 thirtv thin spots may be ob- 

 served. 



In some groups of angio- 

 sperm< the spores of a tetrad 

 do not se[iaratc, a condition 

 once described as a compound 

 grain. In certain cases still 

 larger groups of spores cling 

 together, and this tendencv 

 reaches its extreme expression 

 in such plants as orchids and 

 milkweeds, where all the 



