13 
species eventually break through it, forming. superficial oil vesicles. 
Anthriscus and some Bupleurums have neither^ oil-tubes nor 
secreting cells. In Osmorhiza the mature fruit usually gives no 
trace of oil-tubes, while the immatui'e fruit may show groups of 
4 to 6 oil-tubes in the intervals, and 1 to 3 in the ribs themselves. 
The obliteration of oil-tubes on approach to maturity may 
account for the discrepancy of opinion concerning the oil-tubes of 
certain genera. It is an interesting fact that this suppression of 
oil-tubes seems to involve the formation of an oily layer. In 
Osmorhiza the mature fruit has an oily layer about the seed, a 
region occupied in the immature fruit by a distinct line of oil-tubes. 
In Hydrocotyle^ however, the isolated groups of secreting cells 
are such as always precede the formation of oil-tubes. -In Hydro- 
cotyle, therefore, the oil-tubes could be called rudimentary ; in most 
Umbelliferce\\\Q.y are oil-tubes proper; in Conium a secreting layer 
has been developed ; while the development of the Osmorhiza fruit 
gives us distinctly all three phases. Hence, to summarize: (1) 
most genera have distinctly developed oil-tubes, various!} placed ; 
(2) a few have a layer of secreting cells; (3) some have groups of 
secreting cells; and (4) others have neither oil-tubes nor secreting 
cells, in fact, without any representation of this oiie of the three 
structures of the mesophyll. 
Strengthening cells. Under this name we would define cer- 
tain groups of cells w'hich are unlike enough in structure, but 
seem to serve the common purpose of strengthening the pericarp 
wall or its ribs. They usually occur beneath each rib, and are 
normally developed about the simple fibro-vascular elements of the 
pericarp wall. These fibro-vascular elements may eventually be- 
come obliterated. The group of strengthening cells may consist 
of fibrous tissue, sclerenchyma, sclerenchymatous parenchyma, or 
small-celled parenchyma. It may be well marked off from the 
surrounding tissues, as in the distinct thick-walled groups of 
Osmorhiza., Cryptotcenia., Conium and ChcErofhyllum \ or it may 
gradually merge into the surrounding tissue, as in Angelica., 
Thaspium., etc. In Hydrocotyle., Bowlesia.^ Pastinaca., and 
Heracleum., the strengthening cells are developed in a broad con- 
tinuous band about the seed-cavity, and so completely make up 
the pericai'iD wall in Bifora and Coriandrum as to make it crus- 
taceous; while in Sanicula., Conioselinum., and yBthusa they seem 
