ANATOMY OF TISSUES. 93 
tissue and it also connects with the epidermal system without 
air-spaces, forming in this respect a very sharp contrast to the 
spongy tissue. This (or the latter tissue) lies below the palisade 
cells and in connection with the lower epidermis. Between the 
palisade and spongy tissues run the bundles of the vascular 
system, which form the framework of the leaf. The spongy 
tissue is hike the mesophyll of the cryptogamic leaf, except that 
its cells are more irregular in shape. They often send out arms 
in different directions between which are the large air-spaces 
peculiar to this tissue, and continuous with those of the numer- 
ous stomata. There is also frequently a hypoderma or layer 
of cells immediately under the epidermis, whose office is to 
support the epidermal cells. In certain fleshy plants this hypo- 
derma consists of several layers and is supposed to act as a 
water reservoir. Examples of this are leaves of Ficus and 
Tradescantia. 
Nervature. — The ridges or nerves evident on nearly all leaves 
are formed by the strands of bast or collenchymatic tissue which 
occur usually in clusters on the under and upper sides of the 
bundles. The nervature therefore coincides with the course of 
the bundles. These are branched in all the phanerogams, 
though for systematic purposes some are described as unbranched. 
Even in the parallel nerves of the monocotyledons minute 
branches stretch out from the large ones, anastomosing with 
them and with each other. 
Through the petiole the bundles generally extend straight 
toward the blade, and are arranged either in an arc open toward 
the upper surface of the leaf, or in a closed ring, or sometimes 
S 
with no order but scattered irregularly over a cross-section of 
the petiole. In the blade they spread out in different directions 
branching and anastomosing, their course being especially varied 
in dicotyledonous leaves. The two parts of the bundles are so 
arranged that the phloem is toward the upper, the xylem to- 
ward the under surface of the leaf. The bundles either end free 
