. Morphology of gymnosperms. Gymnosperms; Plant morphology. 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
. Morphology of gymnosperms. Gymnosperms; Plant morphology. 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 i
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