i8 THE STRUCTURE OF ECONOMIC PLANTS 



or vessel segments, which are joined end to end with perforate 

 division walls. " The vessel segment may be distinguished from a 

 tracheid in two important respects: (i) it has a definite end wall; 

 and Ql) this end wall is perforated in some manner. The perfora- 

 tions may be scalariform-porous, oblique-porous, or transverse- 

 porous; and this sequence represents in a general way the probable 

 phylogenetic order in which the perforations of the end walls of 

 the vessel segments have evolved. The situation in herbaceous 

 dicotyledons is in accord with this generalization; for, in most 

 cases, the vessel segments of the secondary xylem are relatively 

 short, with transverse end walls and perforations of the transverse- 

 porous type. In many angiosperms, the terminal perforation 

 consists of a single large opening; and the end wall may be so 

 completely resorbed that there is no evidence of it except for a 

 slightly projecting ring between adjacent vessel segments. (Fig. 



lo, C.) 



The vessel segment may be narrow and elongated, but frequently 

 the cells are broader than tall. The former condition is common in 

 the loose spiral and annular vessels of the protoxylem, while vessels 

 which are differentiated and matured after the elongation of the 

 axis is complete may have broad segments that are short, cylindri- 

 cal, and barrel-shaped. Bailey and Tupper (6), in a comprehensive 

 series of measurements of tracheids and vessel segments, found that 

 the former are usually considerably longer than the latter; and 

 that there is a definite correlation between vessel segment length 

 and the character of the perforations. In ±-/6 species of dicotyle- 

 dons the average vessel segment length was 0.57 mm., while the 

 average length of the tracheids in 146 species of conifers was 3.64 

 mm. In X74 species of dicotyledons, 168 had transverse-porous 

 perforations; and 138 were 0.3 to 0.6 mm. or less in length. They 

 found further that vessel segments are shorter in the more slowly 

 growing plants; but noted that length is also dependent upon the 

 length of the cambial mother cell and the amount of elongation 

 that the vessel segment undergoes during differentiation. The 

 lateral walls of the vessel segment correspond to those described 

 for the tracheid; and there may be spiral, annular, scalariform, 

 reticulate, and pitted vessel segments. 



The Ontogeny of the Vessel. — There are several ways in 

 which a vessel may develop, the most common one being that 

 described by Fames and MacDaniels (15), in which the vessel 



