404 



A TEXT-BOOK OF BOTANY. 



chromoplastids, or, like the epidermal cells, a colored sap ; in some 

 instances, as in the buttercups, starch grains are also found in 

 the mesophyll. Calcium oxalate crystals are usually present, and 

 milk vessels are sometimes found, as in the Papaveraceae. 



The FILAMENT and connective possess a central nbrovascular 

 bundle, around which are arranged comparatively small paren- 

 chyma cells and among which secretion cells are sometimes scat- 

 tered, as in Tilia. The pollen sacs consist of but two layers of 

 cells an outer layer called the "exothecium," which resembles 

 the epidermis of the corolla, and an inner layer called the " endo- 

 thecium," the cells of which are contractile and peculiarly thick- 

 ened, this feature being rather characteristic for certain species 

 (Fig. 232, 7). Lining the pollen sacs during their development, 



c 



FIG. 233. Several forms of pollen grains: A, crocus; B, arnica, with three thin places 

 in the wall through one of which the pollen tube may protrude; C, lavender showing six 

 thin places in the wall. 



there is a layer of cells, called the " 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 (Fig. 233, A) ; more or less three- 

 sided, as in the Composite and in cloves ; four- or five-sided, as in 

 Viola tricolor (Fig. 232, K, L, M), and in some cases, as in the 

 Pinaceae, they may be winged. In addition to protoplasm and 

 one or more nuclei, pollen grains contain considerable oil and 

 starch. The outer or enclosing membrane (Fig. 233) consists of 

 two parts : an inner one, known as the " intine," and consisting of 

 cellulose, and an outer, known as the * exine," apparently con- 

 sisting chiefly of cutin ; in some cases the exine also contains an 

 oil which is colorless, as in Salvia, or yellowish, as in lavender, 

 and in some instances it may contain a viscid substance, causing 



