12 
wings longitudinrdly placed. The two nearest the commissure 
are the laterals \ a single dorsal one occurs on the back of the 
carpel, while the pair between the dorsal and the laterals are the 
intermediates , In addition to these fiv'^e primary ribs or wings 
secondary ones may appear (as in Hydrocotyle Asiaticd), and even 
become more prominent than the primary ones (as in Daucus and 
Trefocarpus)'^ or occasionally even all external indications of ribs 
may be lacking (as in Sanicula'). The presence of these ribs or 
wings, their varying size and structure, furnish good diagnostic 
characters. . , ^ , 
The general structure of the pericarp wall well represents the 
typical leaf structure, with under and upper (that is, outer and 
inner) epidermal layers bounding a more or less developed 
mesophyll. It is in the mesophyll region that the characteristic 
fruit structures are developed. The constant occurrence in the 
outer epidermal region of the two distinct layers of epidermal 
.cells suggests that the outer layer represents the connate calyx, 
while the inner is the true epidermis of the pericarp. . The 
mesophyll region of the pericarp is naturally separated into three 
structures: (1) oil-ducts, (2) strengthening cells, and (3) undiffer- 
entiated parenchyma. 
/ Oil-tubes. These occur in varying number, size and position 
in the different genera, and, are of such constancy as to furnish 
most valuable characters. By far the. most common position for 
oil-tubes is in the intervals between the ribs, where they occur 
singly or in groups. In this position they may occur close against 
the inner epidermal layer, or centrally in the pericafp section, 
while in Cryptotcenia and Erigenia they seem to be developed 
in the inner epidermal layer itself. In ^Ethusa., Ccelopleurutn 
and Cryptotcenia oil-tubes occur both in the intervals and beneath 
the ribs. In some cases, as in Polytcenia., there are two sets of 
oil-tubes, one forming an almost continuous layer about the seed- 
cavity, the other composed of smaller tubes, and scattered through 
the very thick pericarp. In Zizia and Aletes there are also 
smaller accessory tubes in the ribs. In Conium there is no de- 
velopment of oil-tubes, but the whole' inner epidermal layer of the 
pericarp becomes a secreting layer. In Hydrocotyle., on the other 
hand, in the absence of oil-tubes, groups of secreting cells occur 
just beneath the outer epidermis of the pericarp, and in some 
