ECONOMIC WOODS OF THE UNITED STATES 15 



simple and sometimes, especially in strongly inclined division 

 walls, scalariform, that is, the opening is crossed with few to many 

 bars in ladder-like arrangement. The bars are usually transverse 

 to the longitudinal axis of the vessel. Both simple and scalari- 

 form perforations may occur side by side in the same wood, but 

 usually one form prevails. These features have considerable 

 diagnostic value. For example, the perforations are simple in 

 Acer, but scalariform in Betula and Cornus; in &sculus and Tilia 

 they are mostly simple, but in Liriodendron and Magnolia scalari- 

 form, except in Magnolia acuminata. 



Other characteristics of the vessels are the markings on their 

 walls. In most cases they are abundantly pitted with bordered 

 pits, except in contact with parenchymatous cells where the 

 pitting may be either simple or bordered. (See PITS.) It is very 

 common for vessels, particularly the small ones, to be marked with 

 spirals on their interior walls (e.g., Acer, Ilex, Tilia, Ostrya, &s- 

 culus). In Liquidambar only the pointed ends of the vessel seg- 

 ments are marked with spirals. 



The function of vessels is to facilitate the ascent of water in 

 the stem. Vessels lose their protoplasmic contents by the time 

 their perforations are complete and become filled with air and 

 water, or air alone. When their activity as water-carriers lessens 

 they frequently become plugged with outgrowths from adjoining 

 parenchymatous cells. (See TYLOSES.) In the heartwood of 

 certain species (e.g., Gymnocladus, Gleditsia, Guaiacum, Prosopis) 

 they become wholly or partly filled with gums or resins; in 

 others, with carbonate of lime. 



The length of vessels is usually very great, and doubtless often 

 equals that of the whole plant. In width vessels exhibit great 

 variation not only in different species, but also in different portions 

 of the same growth ring. In some woods all of the vessels are 

 small (e.g., Tilia, JEsculus [Plate VI, Fig. 5], Populus, Salix); 

 in others they are mostly large (e.g., Juglans); very often, as in 

 Quercus (Plate II, Figs. 5, 6), they vary from large (0.6 mm.*) 

 and conspicuous to very small (0.1 mm.). 



Vessels in cross section are called pores, and when this term 

 is employed it will be understood to apply to cross sections 

 exclusively. Pores are usually readily distinguishable from the 

 adjoining elements by their larger size, though it is not always 



* One millimetre is equal to about one twenty-fifth of an inch. 



