464 PART III. THE CLASSIFICATION OF PLANTS. 



those of the vegetative shoots, as in Dacrydium cnpressinum, where 

 the leaves of the fertile shoots are scale-like and radially arranged, 

 whilst those on the vegetative shoots are linear and are arranged 

 in two lateral rows ; or as in Sequoia gigantea where the leaves of 

 the fertile shoots are shorter, broader, and more closely arranged 

 than of the vegetative shoots : or the younger leaves may differ 

 from the older, as, for instance, in certain Cupressineee with scale- 

 like foliage-leaves, where the first foliage- leaves of the young 

 stem are acicular (Thuja), or where some of the branches bear 

 .acicular leaves (Juniperus) : or in these same Cupressineae (e.g. 

 Thuja) the leaves borne on the flanks of the shoot differ in 

 shape from those on the upper and lower surfaces, the latter 

 being further distinguished by having each a resin-receptacle (see 

 Fig. 306, p. 486). 



Scale-leaves, destitute of chlorophyll, occur in nearly all the 

 Cycadaceae, in most Conifers (absent in most Cupressinese and 

 Araucariese) and in Ephedra (Gnetaceae). In the Cycadaceae the 

 scale-leaves are present in great number, completely covering the 

 .surface of the stem, and are developed at the growing-point alter- 

 nately with the foliage-leaves, bat in much greater numbers: in 

 Pinus, Phyllocladus, and Sciadopitys, the scale-leaves are the only 

 leaves borne by the long shoots; in Phyllocladus and Ephedra 

 they are the only vegetative leaves. 



The Primary Root always persists as a tap-root. 



General Histology. The Stem. The growing-point of the stem is 

 generally described as presenting in the Abietiiieas a well-marked 

 differentiation into dermatogen, periblem and plerome, whilst in 

 the other Coniferse the distinction between periblem and plerome 

 is less clear, and in Ephedra even the distinction between derma- 

 togen and periblem is not definite ; it has been asserted from time 

 to time, though without absolute demonstration, that, in the Cy- 

 cadaceae, the Coniferae, and in Gnetum and Ephedra among the 

 Gnetaceas, there is an apical cell, either three-sided or four- sided, 

 in the growing-point. 



The stem is monostelic : the stele is surrounded by a pericycle 

 which may be simply pa,renchymatous (e.g. Pinus) or consist of 

 thick- walled fibres (e.g. Taxus) : the primary vascular bundles are 

 collateral, are open, and have the usual general structure ; they 

 are generally arranged in a single circle round the pith, but in 

 Cycas there is a system of cauline bundles in the cortex, in 

 Encephalartos there is a similar system in the pith, and in Welwit- 



