THE PLANT BODY 45 



side walls (Fig. 21L). The perforations were reduced in number, en- 

 larged, and fused, by loss of the ladderlike bars between them, to 

 form a single opening. Vessel elements with one large pore in each end 

 wall are termed simply perforate (Fig. 21G to L); elements of less spe- 

 cialized type, with two or more perforations on the end walls, are termed 

 scalariform or midtiperforafe (Fig. 21A to C). Modification in form of 

 vessel element is phylogenetic, but perforation of the wall is ontogenetic. 



In primitive, vessel-bearing woods, the vessels are solitary or chiefly 

 so (Fig. 22); in advanced types, they tend to be aggregated. Where 

 the vessels, solitary or clustered, are scattered through the annual ring 

 — as seen in cross section — the wood is disuse-porous; where the vessels, 

 especially the largest ones, are chiefly in the early wood of the annual 

 rings, the wood is ring-porous. Ring porosity is an advanced character 

 associated usually with simply perforate vessels. 



Scalariform vessels occur in about 110 families — exclusively in some, 

 dominantly in others, infrequently or sporadically in others. Except for 

 the Paeoniaceae, whicli includes both herbaceous and woody species, 

 families in which scalariform vessels occur exclusively are all woody — 

 Eupomatiaceae, Himantandraceae, Annonaceae, for example. Families 

 in which part of the vessels are scalariform are scattered tlirough the 

 dicotyledons — Cornaceae, Betulaceae, Ericaceae, Bixaceae, Magnolia- 

 ceae, Theaceae. Scalariform vessels are most common in families recog- 

 nized as primitive in other characters, especially those that seem to be 

 primitive members of more or less isolated lines — Magnoliaceae, Betula- 

 ceae, Ericaceae. They sometimes accompany advanced flower structure 

 — Betulaceae, Caprifoliaceae. Simple vessels are also broadly distributed 

 among angiosperms. The vessels of herbs, even in primitive families like 

 the Ranunculaceae, are simply porous. (Paeonia, with scalariform ves- 

 sels, has been removed from the Ranunculaceae.) 



Vessels are absent in some aquatics — Ceratophyllaceae, Nymphae- 

 aceae, Podostemonaceae — parasites, saprophytes — Monotropa; in these 

 taxa, absence clearly represents phylogenetic loss. Throughout angio- 

 sperms, in highly specialized secondary wood, vessel number tends to be 

 greatly reduced. Some annual dicotyledonous herbs have few or no 

 vessels, as do some larger plants of unusual habit, such as the Cacta- 

 ceae and larger Crassulaceae, which possess strong vascular cylinders. 



The vessel has arisen independently in most of the major lines of 

 vascular plants: Selaginellales — SelagineUa; Equisetales — Equisetum; 

 Gnetales (sensu stricto) — Gnetum; Welwitschiales — Welwitschia; Ephe- 

 drales — Ephedra; Dicotyledoneae — apparently at least several times; 

 Monocotyledoneae — many times. The vessel cannot be considered a 

 distinguishing character of the angiosperms. That some angiosperms 

 are vesselless was known as early as 1842; in 1900, the "Homoxyleae," a 



