496 BOTANICAL GAZETTE [june 



season, and that the maturity of the secondary shoot with the 



■ 



ovule results in the death of the primary shoot as well. This is 

 not the normal situation, as usually only the secondary shoot 

 with the ovule drops from the primary shoot, which remains in the 

 axil of the leaf, a branch scar showing the place of detachment 

 at the secondary shoot from the primary shoot (fig. 6). Detach- 

 ment of the secondary shoot is probably accomplished normally 

 by the formation of an absciss layer across the base of the shoot. 

 The region of abscission is marked by a narrow layer of platelike 

 cells, rich in protoplasm, outside of which is a layer (5-6 cells wide) 

 of cork tissue, and whose outer border consists of radially elongated 

 cells which form a conical cap to the scar (fig. 7). When collec- 

 tions of T. canadensis for this study were first begun, in the autumn 

 of 1 91 3, it was noticed that ovulate buds were to be found on older 

 as w r ell as on the current season's growth, as has since been pointed 

 out for T. baccata by Miss Aase (i). This is not due to dormancy 

 of buds which had failed in development, as might usually be 

 assumed, but to the persistence of the primary shoot year after 

 year, producing one or two new secondary shoots each season. 

 This renewal of growth is contemporaneous with that of the 

 primary shoots of new branches, beginning early in the spring, 

 although not becoming recognizable externally until later in the 

 summer, when it can be distinguished by the slight projection which 

 appears at the base of the secondary shoot (fig. 8). Growth is 

 slow, and by the middle of July is arrested, as in previous seasons, 

 by the growth of the new secondary shoot (fig. 9). As these 

 observations show, the primary shoot is a persistent structure and 

 may produce secondary shoots season after season, or become a 

 leafy shoot, the situation being evidence against regarding the 

 primary shoot w r ith its secondary shoot as representing a compound 

 strobilus. 



Terminal primary shoot. — Several cases were found in which 

 the primary shoot was a terminal structure of the leafy branch 

 (figs. 10, 11), the terminal bud having developed as a primary 

 ovuliferous structure, bearing a secondary shoot. That this may 

 continue to function as a primary shoot for more than one sea- 

 son is shown by the presence of a secondary branch scar a little 



