48z THE STRUCTURE OF ECONOMIC PLANTS 



some thickening of the xylem parenchyma on the adaxial face 

 of the bundle. The main bundles extend to the margin of the 

 leaflet, where they terminate in conical projections, and the 

 secondary veins form a reticulum with many cross-connecting 

 veinlets. 



Floral Development. — The floral development of the Umbel- 

 liferae received early attention by many workers, including Payer 

 (i8), von Mohl (15), and Sieler (ii). More recently, the subject 

 has been investigated by Jurica (ix) and Beghtel (4), the latter 

 account dealing specifically with Pastinaca, which is similar to 

 Apium in its floral ontogeny. The umbellets of the compound 

 umbel develop centripetally so that the centrally located ones are 

 the last to mature. This is also true of the flowers in each umbellet, 

 where the peripheral ones are the first to develop. (Fig. i.^j, C, D.) 

 The flower primordia arise from the broadened apex of the umbellet 

 as club-shaped structures. The apex of each pedicel broadens and 

 the primordia of the petals, stamens, and carpels arise in acropetal 

 succession. (Fig. i47, £-/.) The involucel is rudimentary and 

 does not develop, being represented in the young umbellet by a 

 projecting fold of tissue. The flower is epigynous and the calyx 

 is represented by a ridge at the summit of the inferior ovary. 



(Fig- M7, /•) 



The primordia of the two carpels originate as independent 



crescentic structures which unite to form the two-loculed ovary. 



The pendulous, anatropous ovules are initiated before complete 



coherence of the carpels has been effected. In the development 



of the ovule, the nucellus arises from the placenta on the inturned 



edge of the carpel as a club-shaped structure which is at first 



straight. As growth continues, it begins to turn toward the outer 



wall of the ovary until it becomes completely anatropous. The 



funiculus is slender, and a single, thick, fleshy integument develops 



which completely encloses the nucellus. On the side next to the 



funiculus, the integument is scarcely recognizable because of the 



complete anatropy of the ovule. 



LITERATURE CITED 



I. Ambronn, H., "Ueber die Entwickelungsgeschichte und die mechanischen 

 Eigenschaften des CoUenchyms. Ein Beitrag zur Kenntniss des mechan- 

 ischen Gewebesystems." Jahrb. Wiss. Bot. 12: 473-541, 1881. 



z. Anderson, D., ■'Ueber die Struktur der Kollenchymzellwand auf Grund 



