MORPHOLOGY OF HIGHER PLANTS. 



213 



106, B, C) . The mesophyll is made up of rather loose, branching 

 parenchyma cells (Fig. 118, D), with large intercellular spaces. 

 The cells are free from chloroplastids, may contain chromoplas- 

 tids, 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 fibrovascular 

 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- 



FiG. 119. Several forms of pollen grains: A, crocus; B, arnica, with three thin places 

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

 thin places in the wall. 



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

 ened, this feature being rather characteristic for certain species 

 (Fig. 118, /). Lining the pollen sacs during their development, 

 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. 119, A) ; more or less three- 

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

 Viola tricolor (Fig. 118, K, L, M), and in some cases, as in the 

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

 one or more nuclei, pollen grains contain considerable oil and 

 starch. The outer or encjosing membrane (Fig. 119) consists of 

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



