350 MORPHOLOGY OF THE ANGIOSPERMS 



An even number of traces is clearly basic for the angiosperm cotyle- 

 don; it also seems to be the primitive condition. The taxa now generally 

 recognized as primitive — some of the woody Ranales, the lower Liliales, 

 the Helobiales — have cotyledons with two strong traces, often with pairs 

 of lateral ti-aces. An even number of cotyledon traces is in strong con- 

 trast to the odd number characteristic of leaves; even-numbered leaf 

 traces, as in Austrobailei/a (Fig. 5) are rare. The significance of the 

 double nature of the vascular supply of the cotyledon is obscure. In 

 most gymnosperms, even-numbered traces are characteristic, and single 

 traces seem to represent two fused traces. In angiosperms, the odd 

 number of leaf traces is apparently, as with the cotyledons, a deriva- 

 tion from an even number; stamens and carpels occasionally have an 

 even number of traces. The cotyledons retain strong evidence of an 

 ancestral even number. The double vascular supply of cotyledons, to- 

 gether with frequent bilobing, has been looked upon as evidence in the 

 cotyledon of dichotomy surviving as a primitive character in the embryo. 

 But the traces are derived well apart on the hypocotylary stele, and 

 there is no evidence of dichotomy in the embryonic axis. In the simple 

 two-trace cotyledons of the Commelinaceae (Tinantia), the traces arise 

 at various distances apart, even 120°, and are probably never directly 

 opposite one another. This is also evidence against the view that the 

 basic two-trace vascular supply and the frequent bilobing of cotyledons 

 is a retention of ancestral dichotomy. In taxa with more than two traces, 

 the traces are closer together, and two, or even more, may be derived 

 from one gap. 



The number of traces is fairly constant in families. Two traces 

 characterize most of the lower monocotyledon families. The Liliaceae 

 have two in the lower tribes; two, four, and six in others. The Zingi- 

 beraceae have two traces. The Amaryllidaceae have two, or one double 

 trace; the Iridaceae, one, two, or three (Crocus has one in some species, 

 two in others); the Araceae, one to several; the Cannaceae have several. 

 Some of the woody Ranales have two cotyledonary traces. The vascular 

 system of the cotyledon of monocotyledons has, characteristically, two 

 major bundles or two with two additional lateral bundles. The dicotyle- 

 dons have, commonly, three major bundles (the median equivalent to 

 the two median bundles of the monocotyledons). Cotyledons with a 

 pair of midveins are occasional in the dicotyledons — Austrobaileyaceae, 

 some of the Chloranthaceae and Monimiaceae. In many grasses, the 

 single trace breaks into two at the base of the cotyledon and forms a 

 forked midvein — Hordewn; in other genera — Trificum — the median 

 trace is double. 



Anatomy of the Hypocotyl. The gross structure of the hypocotyl needs 

 brief description, because of the occasional statement that the mono- 



