MORPHOLOGY 
FIGS. 575-578. 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 sporogenous 
tissue (after one division of primary sporogenous 
cells) in the mother cell stage; 578, a mother 
cell containing a tetrad. After MERRELL. 
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 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 (inline) 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 
thirty thin spots may be ob- 
served. 
In some groups of angio- 
sperms the spores of a tetrad 
do not separate, a condition 
once described as a compound 
grain. In certain cases still 
larger groups of spores cling 
together, and this tendency 
reaches its extreme expression 
in such plants as orchids and 
milkweeds, where all the 
