EVOLUTION OF ANGIOSPERMS THROUGH APOSPORY 1 75 



The leaf may be considered the most primitive element, and 

 is in most cases also the most essential part of the plant metamer. 

 The joints are often reduced to mere rudiments, and the roots 

 entirely lacking. Sometimes internodes of special stems (root- 

 stocks) are devoted to the production of roots. Sometimes roots 

 produce buds and give rise to new stems or new plant indi- 

 viduals. Thus the many seedless varieties of the breadfruit 

 are propagated from pieces of roots. In Scchimn the primary 

 metamer has no internode-like basal joint (hypocotyl), but many 

 roots grow out independently from the united bases of the 

 cotyledons. Root-hairs grow out in a similar manner from 

 basal cells of the embryos of Podostemonaceae. The cotyledons 

 of Plantago and the leaves of Begonia and Bryophylhun also 

 produce roots. 



In addition to their absorptive functions, root^ often serve 

 as organs of aeration. Even subterranean roots may send 

 branches upward into the open air for this purpose. This occurs 

 not only in the mangroves and other swamp plants, but in cotton 

 and other dry land types. Roots may also be specialized as 

 haustoria in parasites, and as hold-fasts in climbers, or even as 

 spines to afford a protective armature, notably in certain species 

 of yams ' and in the palm genus Acanthorhiza. Although the 

 spines of Acanthorhiza are often large and compound like those 

 of Glcditschia, it is apparent that they are roots, not only because 

 there is a complete series of transitions, but also because of the 

 large deciduous root-caps. 



It is not impossible that even the spines of cacti may prove to 

 be modified roots, instead of leaves. The spines of many species 

 are capped, and their number, position and arrangement are at 

 least as favorable to their being roots as to the usually accepted 

 theory that they represent leaves. The modification that roots 

 would require to become spines is much less Ihan in the case of 

 leaves. Not only in the genus Pcireskia, but in many species 

 of Ofuntia^ true leaves are developed, and it is in the axils of 

 the leaves that the spines arise, just below the point from which 



^Safford, W. E., 1905. The Useful Plants of the Island of Guam. Contrib. 

 U. S. National Herbarium, 9: 69. These spine roots of Dioscorea protect the 

 edible starch-filled tubers. 



