238 



MORPHOLOGY OF GYMNOSPERMS 



centripetal xylem of the ancestral mesarch bundle. The latter cells 

 are said to mediate between the mesophyll and the phloem in the 

 transfer of food. The single vascular bundle as it enters the leaf 

 divides into two strands that run parallel and near together within 

 the common sheath. This is really the double leaf trace characteristic 

 of the more primitive gymnosperms, and separating into its two con- 

 stituents at the leaf base. 



In the case of flat leaves the mesophyll differentiates into the pali- 

 sade and spongy regions, and in the broader forms the cells of the 

 central mesophyll may be more or less transversely elongated, and 

 the entering vascular bundle breaks up into several more divergent 

 strands. 



2. The spore-producing members 



The strobili of Pinaceae are monosporangiate, the two kinds occur- 

 ring on the same plant (Abietineae, Taxodineae, and occasionally 



in Cupressineae and Araucarineae) or 

 on different plants (the prevailing 

 condition in Cupressineae and Arau- 

 carineae) . Bisporangiate strobili occur 

 occasionally, having been reported for 

 Picea excelsa (9), Finns maritima (58), 

 Ahies sp., Pseudotsuga Douglasii (154), 

 and Sequoia (42) (figs. 260-262); 

 and recenlly a remarkable plant of 

 Juniperus communis has been de- 

 scribed (97), which bears such strobili 

 almost exclusively. In this last case, 

 the strobilus consists of a few sterile 

 bracts at base, then two or three 

 whorls of staminate sporophylls, and 

 at the tip the ovulate sporophylls; oc- 

 casionally there is a whorl of sterile 

 bracts between the stamens and the 

 ovules. Such a strobilus is interesting 

 not merely because it is bisporangiate, 

 but also because the relation of the 



Fig. 260. — Bisporangiate stro- 

 bilus of Pinus maritima: the three 

 lowest sporophylls on each side of 

 this section bear microsporangia; 

 the next sporophyll above on each 

 side bears a microsporangium and 

 also a rudimentary ovuliferous 

 scale in the axil; above these 

 there is the structure of an ordinary 

 ovulate strobilus; Goebel found 

 hundreds of such cones upon one 

 tree. — After Goebel (58). 



