THE FLOWER. 
87 
amount, and the mesophyl consists wholly of rather 
loose chlorophyl parenchyma ; the outer or under epi- 
dermis contains the stomata, and if hairs are present, 
they also arise from this surface ; the fibrovascular 
bundles are generally simple in structure, although in 
some cases, as in lavender, sclerenchymatous fibers are 
strongly developed. 
In the corolla the epidermal cells are generally more 
or less centrifugal ly developed, forming prominent 
papillse, which give the petals a velvety or satiny appear- 
ance, as in the rose ; secreting and non-secreting hairs 
are also developed, which are peculiar to the corollas 
of irregular flowers, as in Lavandula vera and Viola 
tricolor; stomata are comparatively few in number. 
The epidermal cells are but slightly cutinized, and 
in surface view are strongly undulate and appear striate 
owing to the papillary development. The mesophyl 
is made up of rather loose, branching parenchyma 
cells, with large intercellular spaces, which are free 
from chloroplastids and may contain, like the epidermal 
cells, a colored sap, or chromoplastids ; in some in- 
stances, as in the buttercups, starch grains are also 
found in the mesophyl. Calcium oxalate crystals are 
usually present, and milk vessels are sometimes found, 
as in the Papaveracece. 
The filament and connective possess a central fibro- 
vascular bundle, around which are arranged compara- 
tively small parenchyma cells and among which secre- 
tion cells are sometimes scattered, 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 
“ endothecium,” the cells of which are contractile and 
peculiarly thickened, this feature being rather charac- 
teristic for certain species. Lining the pollen sacs, 
