ANATOMY OF TISSUES. : 95 
channels, and then a bundle sheath surrounding the two bundles 
which are farther enclosed by colorless parenchymatic cells. 
These two bundles extend the whole length of the leaf and are 
entirely unbranched. The position of phloem and xylem cor- 
responds to that found in the ordinary flattened leaf. 
In the scales of the winter buds, which are metamorphosed 
leaves, we find a similarity in general structure to that of the 
real leaf. The under epidermis has more strongly thickened 
outer walls. Stomata occur only on this surface, and, what is 
peculiar to this kind of leaf, the primary epidermis is some- 
times replaced by periderm, a tissue which will be described 
later on as belonging to secondary growth. For example, Aes- 
culus Hippocastanum. 
Calyx leaves have stomata only on the under side. The 
mesophyll is often divided into palisade and spongy tissue. 
The bundles have mestome strands only, with no bast. 
Corolla leaves have a uniform mesophyll (not separated into 
palisade and spongy tissue). The ducts of the bundles are 
mostly spiral, these together with thin-walled fibrous cells mak- 
ing up nearly the entire bundle. The epithel has very few 
stomata, and its cells are often papillose. The coloring matter 
occurs either in the epithel or parenchymatic cells, or in both, 
and is usually dissolved in the cell sap. Starch grains and cal- 
cium oxalate crystals are of frequent occurrence, and also 
ethereal oil, the last contained in glands, oil channels, or sus- 
pended in little drops in cell sap of epithel or parenchyma or 
both. 
Cotyledons, in seeds containing no albumen, consist of richly 
developed parenchymatic cells, epidermis without stomata, and 
bundles distributed throughout. The contents of the paren- 
chyma are oil, aleurone grains, starch, etc. 
When the function of the ordinary deciduous leaf is com- 
pleted, its separation from the stem is brought about in the 
following manner. A plate of cells at its base, where it joins 
