228 MORPHOLOGY OF GYMNOSPERMS 



there were Picea sitchensis, Tsuga heterophylla, and Thuja gigantea. 

 In the case of the first species ("Sitka spruce"), the largest tree, 68 

 years old, was less than 60 cm. high; another, 86 years old, was less 

 than 30 cm. high, with, a trunk 1.8 cm. in diameter; a third, 98 

 years old, was about 30 cm. high, with a trunk 2 cm. in diameter. 



VASCULAR ANATOMY 



The anatomical structure of the stems of conifers, and especially 

 of pines, has long been a favorite subject of study, and perhaps no 

 vascular cylinder is more familiar (fig. 257). The mass of details 

 thus accumulated need not be presented in this connection, for much 

 of it has not been organized for morphological use. During recent 

 years, however, great progress has been made in such an organization, 

 and it should find its place in any presentation of the morphology 

 of Coniferales. 



A transverse section of the stem shows a comparatively thin cortex, 

 a thick vascular cylinder built up by a persistent primary cambium, 

 and a small pith. These general features are the same as those of 

 Ginkgo, but they are in contrast with the corresponding features of 

 the other groups previously considered. The vascular cylinder is 

 collateral endarch, and all traces of mesarch structure have disappeared 

 from the Pinaceae, so far as investigated, except in the cotyledonary 

 bundles of Juniperus and of certain species of Cupressus (141), and 

 occasionally in the cotyledons of Tsuga canadensis, Pinus Pinea, 

 and P. Gerardiana (154). In Juniperus the mesarch structure is 

 not pronounced, and in the species of Cupressus referred to, it is indi- 

 cated only by one or two centripetal xylem elements. It is to be 

 expected that mesarch structure, at least in the cotyledons, will be 

 discovered in more of the Pinaceae. If the "transfusion tissue," 

 generally present in the leaves of gymnosperms as an auxiliary con- 

 ducting system, is derived from centripetal xylem, as Worsdell has 

 suggested (45), the mesarch condition in this stage may be regarded 

 as characteristic of the foliar bundles in general, but it would be a 

 later stage than true mesarch- 



One of the most significant facts in connection with the vascular 

 cylinder of Pinaceae is the occurrence of leaf gaps (71). The presence 

 of leaf gaps is a constant feature of the Filicales and Ophioglossales 



