46o BOTANICAL GAZETTE [june 



over it 2-4 bordered pits. The latter are somewhat elongated in 

 many cases, and frequently are so closely approximated as to be 

 flattened by mutual contact and to cover nearly the whole of the 

 primary pit areas. This type of tracheary pitting grades into a 

 second type in which the surfaces of the elongated primary pit areas- 

 are only partially covered by circular bordered pits (figs. 24, 25). 

 The latter type, in turn, grades into a third type in which the 

 primary pit areas are not typically scalariform in structure. Cer- 

 tain of the attenuated areas appear to increase in size at the expense 

 of intervening areas, which are either eliminated entirely or persist 

 as constricted areas that are not overlaid by bordered pits (figs. 26^ 

 29, 30). By this process of specialization certain of the primary 

 pit areas become oval or biconvex and less closely approximated. 

 Certain of the Querleisten tend to widen and to become biconcave 

 with forking ends; whereas others are crowded together and appear 

 to fuse to form similar biconcave thickenings (figs. 26, 29, 31). 

 This type of pitting grades into others in which the attenuated 

 areas that are overlaid by bordered pits become more circular and 

 more widely separated, the intervening primary pit areas and 

 Querleisten (except the curved bands that commonly persist above 

 and below the bordered pits) becoming vestigial or obhterated 

 (figs. 27, 28). In certain cases the portions of the middle lamella 

 between widely separated primary pit areas may be uniformly 

 thickened so that the pits appear to be separated by a single, wide 

 dark colored band. In other cases, for example, in the thick- 

 walled fiber-like cells of the so-called summer wood, and in the 

 small tracheids which occur in seedhngs and the first annual rings 

 of stems and roots, the bordered pits frequently tend to be super- 

 imposed over nearly the whole surface of the circular primary pit 

 areas, and the curved Querleisten cling to the upper and lower 

 margins of the bordering areas, or are completely obhterated. 



Such transitions in tracheary pitting have been observed in 

 Larix, Pinus, Abies, Sequoia, and other genera of the Abieteae and 

 Taxodieae, as well as in Taxodium and Ginkgo. The more elongated 

 primary pit areas and the narrower, straighter Querleisten tend to 

 occur in the larger, thinner- walled, heavily pitted tracheids; and 

 are therefore most characteristically developed in the first formed 



