466 PART III. THE CLASSIFICATION OF PLANTS. 



and is surrounded by a well-marked endodermis ; in the other genera 

 it appears to be specially differentiated mesophyll frequently (e.g. 

 Cycas, Podocarpus, Cephalotaxus) consisting of cells elongated 

 transversely to the long axis of the leaf : the use of the trans- 

 fasion-tissue is to compensate for the absence of a much-branched 

 Vascular system in the leaf, the tracheidal cells serving to distri- 

 bute water from the xylem of the bundles to the mesophyll, the 

 other cells serving to convey organic substances formed in the 

 mesophyll to the phloem of the bundles. 



The Root grows in length by means of a growing-point differen- 

 tiated into dermatogen, plerome and periblem, and root-cap as in 

 Dicotyledons (see p. 154) ; there are commonly two xylem-bundles 

 in the stele: the cambium-ring is formed in the usual way: the 

 phellogen is derived from the pericycle ; in many cases the cortical 

 cells, other than those of the endodermis, show thickenings on 

 their radial walls similar to those of the endodermis-cells ; this is 

 either confined to a single (penultimate) layer of the cortex (e.g. 

 Cupressus, Thuja, Biota, Taxus, Cephalotaxus, Ginkgo), or it 

 extends to several layers (e.g. Sequoia, Taxodium, Juniperus, 

 Araucaria) : the epiblema is generally devoid of root- hairs, but 

 these are abundant in Taxus. 



General histological peculiarities. In all the Coniferae, except 

 Taxus, resin-ducts (see p. 139) are present: they are always to be 

 found in the leaves and in the cortex of the stem, sometimes also 

 in the pith of the stem (Ginkgo), in the primary wood (Pinus, 

 Larix), or in the primary bast (Araucaria); they are absent from 

 the root in many genera (Cryptomeria, Taxodium, Podocarpus, 

 Dacrydium, Tsuga, Cunninghamia, Ginkgo), and when present 

 they never occur in the cortex, but are situated in the primary 

 wood (Pinus, Larix), in the primary bast (Araucaria), or as a single 

 canal in the centre of the conjunctive tissue (Cedrus, Abies, Pseu- 

 dolarix) : they are formed also in the secondary wood (Pinus, 

 Picea, Larix) or in the secondary bast (Cupressus, Thuja, Arau- 

 caria) of both stem and root. Mucilage-ducts, resembling the 

 resin-ducts of the Coniferae, occur in the cortex of the stem in the 

 Cycadaceae. 



The bast of the Gymriosperms resembles that of the Pterido- 

 phyta, and differs from that of the Angiosperms, in that it contains 

 no companion-cells (see p. 137), the function of these cells being 

 performed by certain cells belonging either to the medullary rays 

 (Abietineae, some Cupressineae and Taxodineae) or to the bast- 



