464 MORPHOLOGY OF THE ANGIOSPERMS 



terpreted as parallel groups, derived from the same ancestral stock. The 

 presence of both woody and herbaceous forms in the primitive families 

 of angiosperms is the basis for tlie theory that there were, probably, in 

 the ancestral stock of angiosperms, or in the earliest angiosperms, both 

 woody and herbaceous types. (The division of the dicotyledons into the 

 Lignosae and Herbaceae, setting apart two major lines, based, in part, 

 on habit, seems doubtfully valid as a basis for separating natural taxa. 

 Much more information from various fields of study is necessary to 

 determine the relations of the more primitive families.) 



The monocotyledons may have been derived from dicotyledonous 

 stock early in the history of the angiosperms. The presence of ap- 

 parently only distantly related or unrelated lines among the more 

 primitive orders — Butomales, Alismatales, Helobiales, Liliales — suggests 

 independent origin of these orders. Variety of habit and structure in 

 the primitive families suggests a very long period of specialization since 

 angiosperms arose. If the monocotyledons arose from early dicotyle- 

 donous stock, the origin must have been before vessels arose in dicotyle- 

 dons, because vessels have been shown to have arisen at least several 

 times in the monocotyledons. 



The Stachyospory-Phyllospory Theory. On the basis of supposed 

 differences in ovule position — borne on stem (stachyosporous) or on 

 leaves (phyllosporous) — the angiosperms are divided, under this theory, 

 into two new major divisions — the Phyllosporeae and the Stachyosporeae 

 (see Chaps. 6 and 7). By this grouping, the monocotyledons and di- 

 cotyledons and some families, even some genera, are divided. To cover 

 taxa so divided, a third group was later proposed, one intermediate be- 

 tween the two. The theory is valueless, because comparative morphology 

 and anatomy show that, in the angiosperms, all ovules are borne on 

 appendages. 



Origin of the Angiosperms 



The problem of tlie origin of the angiosperms has received most 

 attention since the beginning of the twentieth century, as increasing 

 information made older theories untenable. Several major taxa were 

 formerly suggested as possibly ancestral to the angiosperms. 



The Isoetes-Monocotyledon Theory. When the monocotyledons were 

 generally accepted as the most primitive angiosperms, resemblances 

 were seen in the linear leaves and cormous habit of Isoetes to the 

 simpler, aquatic monocotyledons. But the microphyllous vascular crypto- 

 gams seemed a most improbable ancestral stock. 



The Coniferales-Amentiferae Theory. Attention then turned to the 

 highest gymnosperms. Resemblances of the angiosperms to the conifers 

 were seen in the Amentiferae, considered in prominent taxonomic treat- 



