420 PART IV. CLASSIFICATION. 



entiation into dermatogen, periblem and plerome. The stem is 

 monostelic : the primary vascular bundles are collateral, are open, 

 and have the usual general struct are ; they are generally arranged 

 in a single circle round the pith. Secondary growth in thick- 

 ness takes place as a rule by means of a normal cambium-ring. 

 In the Cycadacese and Couiferse, the secondary wood consists 

 exclusively of tracheides with rounded or elongated bordered pits 

 and of parenchymatous medullary rays, but true vessels are formed 

 in the Grnetacese ; the secondary bast has generally the normal 

 structure, but in some cases (Abietinese) it has no bast-fibres. 



The Foliage-leaf is characterised by its well-developed epidermis 

 the cells of which are fibrous (Pinus, Torreya) : the stomata 

 are always depressed below the surface, and are borne usually on 

 the under surface only, when the leaf is flat (e.g. Abies, Taxus, 

 Cfiiikgo, etc.), or on the upper side only (Juniperus), but on all 

 sides when the leaf is acicular (e.g. Piuus, Picea, etc.) : the 

 epidermis is supported by a hypodermal layer of fibrous scleren- 

 chymatous cells : when the leaf is flat, the mesophyll is more or 

 less clearly differentiated into palisade and spongy tissue, but when 

 it is acicular, the mesophyll is uniform throughout, consisting of 

 parenchymatous cells with curiously infolded walls (Fig. 93, p. 114) : 

 the acicular leaves (Abietinese) have a single central vascular 

 strand enclosing two bundles which give off no branches : in the 

 flattened leaves there may be several ribs which either do (e.g. 

 Grinkgo) or do not branch in the lamina, and in all these cases the 

 bundles end blindly. A remarkable feature in the structure of 

 the leaf is the presence, in all the genera, of a tissue, termed 

 transfusion-tissue (p. 118), which consists of parenchymatous 

 cells, some of which contain no protoplasm and have pitted walls, 

 being in fact tracheides, whilst others contain protoplasm and have 

 unpitted walls. The use of the transfusion- tissue is to compensate 

 for the absence of a much-branched vascular system in the leaf, 

 the tracheidal cells serving to distribute 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. 103) ; 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. 



