88 
PLANT MORPHOLOGY. 
during their development, there is a layer of cells, the 
so-called “tapetal cells;” but these are usually sooner 
or later absorbed. 
The pollen grains vary greatly in number, as well as 
in size and shape. They are usually more or less 
ellipsoidal but may be spherical, as in Crocus ; more or 
less three-sided, as in the Composite and in cloves ; four 
or five-sided, as in Viola tricolor, and in some cases, as in 
the Coniferae, they may be winged. Pollen grains con- 
tain protoplasm, one or more nuclei and considerable 
oil and starch ; the covering or inclosing membrane 
consists of two parts, the inner of which is known as 
the “intine” and consists of cellulose, and the outer, 
known as the “ exine,” apparently consists chiefly of 
cutin ; in some cases the exine also contains an oil 
which is colorless, as in Salvia, or yellowish, as in lav- 
ender, and in some instances it may contain a viscid 
substance, causing the pollen grains to adhere, as in 
CEnothera. The grains may be smooth or variously 
sculptured ; in most instances the exine is unevenly 
developed, leaving thin places through which the pollen 
tubes protrude in germination ; these give the appear- 
ance of grooves when the grains are dry, and the num- 
ber of grooves are characteristic for different species ; in 
most of the Composite they are three in number ; in the 
Labiate there are six, while in Crocus they are wanting. 
The epidermal cells of the stigma are quite character- 
istic: the cells of the epidermis, or so-called “stigma- 
epithel,” may be palisade-like, forming a more or less 
wart-like mass as in the viscous stigmas of the Umbellif- 
erae, or the outer walls may be modified to rather long, 
broad papillae, as in matricaria and arnica, or they may 
be developed into hair-like processes, as in Crocus. The 
germinating pollen tubes enter the style through an 
open canal, as in the violets, or they penetrate into th e 
