136 



ECONOMIC WOODS OF THE UNITED STATES 



more than 100. Within the same species, however, the variation 

 is within narrower limits, though the number is never constant. 

 In Magnolia and Liriodendron, for example, the number of bars is 

 usually less than 15 and the spaces are wide, while in Ilex, Nyssa, 

 and Liquidambar the bars are nmch more numerous and are closely 

 spaced. 



In the following table are listed the genera of native woods in 

 which the vessel perforations are exclusively scalariform. In a 

 few cases, perhaps, simple perforations will occasionally be found 

 in association with the predominant type. The list is believed to 

 be complete so far as the trees are concerned but not for the shrubs. 

 Eight of the genera yield wood of commercial importance. It will 

 be noted that no ring-porous wood is included, Scalariform per- 

 forations are never found in large vessels, such for instance as are 

 individually distinct to the unaided eye, presumably because the 

 presence of gratings would interfere with the function of large 

 vessels, namely, the rapid conduction of water in quantity. Solere- 

 der calls attention to "the striking fact that the occurrence of 

 scalariform perforations in the vessel often goes hand in hand with 

 small lumina and the presence of bordered pits on the prosen- 

 chyma." (Systematic Anatomy of the Dicotyledons, p. 1138.) 



TABLE VIII 

 Indigenous Woods with Vessel Perforations Exclusively Scalariform 



Aquifoliace.e 



Ilex 

 Betulace^ 



Bctula 



Alnus 



Corylus 

 Caprifoliace^ 



Viburnum 

 Cornace^ 



Cornus 



Nyssa 

 Cyrillace.e 



Cyrilla 



Ciiftonia 

 Ericace.e 



Rhododendron 



Kalmia 



Vaccinium 



Andromeda 



HAMA.MELlDACEiE 



Liiiuidntnbnr 

 Hamamelis 



* Some simple perforations 



Magnoliace.e 



Liriodendron 

 Magnolia (mostly) 



MYRICACEiE 



Myrica* 

 RhizophoracejE 



Rhizophora 

 Saxifragace^ 



Philadelphus 



Hydrangea 



Ribes 

 Symplocace^ 



Symplocos 



STAPHYLEACE.E 



Staphylea 

 Stvrace^ 



Mohrodendron 

 Theace.« 



Gordonia 



present in Mynai califoTmca. 



