THE ANGIOSPERMAE 



1591 



with no midrib, as evidence of its double nature. The first foliage leaf 

 always has a midrib, usually with lateral bundles as well, while a midrib 

 is always present in the cotyledons of Dicotyledons. The genus Nelumbo 

 (Nymphaeaceae) has been for long a subject of interest because the coty- 

 ledons arise in the embryo as a single ring around the apex, the ring 

 becoming two-lobed later. Is this evidence of the origin of two cotyledons 

 by the division of a primordial one, or is it, as seems more probable, simply 

 a congenital union of two? It would violate all probability to remove this 

 and other pseudo-monocotyledons from the families to which they plainly 

 belong and transfer them to a place among the Monocotyledons. Much 

 more readily will we accept the idea that one cotyledon may arise from the 

 fusion of two. 



The lack of normal secondary thickening among Monocotyledons is, 

 in quite another direction, evidence of their derivative status, since tem- 

 porary bundle cambia have been found in the seedlings of a number of 

 genera: Yucca, Typha, Fritillaria, Zea and Miisa, among others. Some 

 additions are made to the vascular elements from these cambia for a short 

 time, but the cambium is subsequently distorted and lost. 



We have referred above to two possibilities regarding the relationship 

 of the one cotyledon to the two, assuming that the two conditions are 

 indeed related. Mrs. Arber has presented a third possibility. It is difficult 

 to do justice in a few lines to her thesis, which is fully and elegantly argued 

 in her book, "Monocotyledons". She dismisses the argument for the 

 double nature of the single cotyledon from its double vascular supply by 

 pointing out that recent research has shown that the midrib of each coty- 

 ledon in Dicotyledons is also very often a double structure. This, she 

 argues, arises from the nature of its vascular supply, which comes from 

 the primary root, instead of, as in epicotyledonary leaves, from the stem. 

 The primary root is usually either diarch or tetrarch and the " double 

 bundle " of the cotyledon is equated to one pole of the root structure, a 

 xylem between two phloems, with some additions to the xylem and its 

 separation into two portions, very often associated with the disappearance 

 of the protoxylem. She emphasizes that the cotyledons, despite their 

 special name, are essentially the first leaves and concludes that, as the 

 growth rhythm in Monocotyledons is such as to produce only one leaf, 

 with a sheathing base, at each node, there is no need to assume that the 

 cotyledonary node would be difi:'erent, or that vestiges of a second cotyledon 

 need be there in any shape or form. 



We see the monocotyledonous seedling at its simplest in Naias (Fig. 

 145 1). The embryo is spindle-shaped, the upper half being the cotyledon 

 and the lower half the hypocotyl. The plumule is entirely enclosed by the 

 base of the cotyledon. There is no endosperm, so there is no haustorial 

 modification of the cotyledon, which forms a straight green leaf. The 

 development of the primary radicle comes after germination, but it is pre- 

 ceded by a collar of root hairs, markings its base, a feature which is repeated 

 in many other seedlings. Seedlings of the Alismaceae, Juncaginaceae 

 s* 



