12 



wings longituclin;.lIy 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 five primary ribs or wings 

 secondary ones may appear (as in Hydrocotyle Asiatica)^ and even 

 become more prominent than the primary ones (as in Daucus and 

 Trepocarpus); or occasionally even all external indications of ribs 

 may be lacking (as in Sa?iiciila). 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 pericarp section, 

 while in Cryptotcenia and ErigQnia they seem to be developed 

 in the inner epidermal layer itself. In ^"^tJuisa., Ccelopleiirum 

 and Cryptotcenia oil-tubes occur both in the intervals and beneath 

 the ribs. In some cases, as in Polyt(Enia^ there are two sets of 

 oil-tubes, one forming an almost continuous layer about the seed- 

 cavity, the other composed of smaller t'jbes, and scattered through 

 the very thick pericarp. In Zizia and Aletes there are also 

 smaller accessory tubes in the ribs. In Coniiim 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, groujDS of secreting cells occur 

 just beneatii the outer epidermis of the pericarp, and in some 



