GROUP IV. — PHANEKOGAMIA: GYMNOSPERMiE. 465 



schia there are apparently both a cortical and a medullary system. 

 Secondary growth in thickness takes place as a rule by means of 

 a normal cambium-ring ; but in some cases the activity of the 

 normal cambium is short-lived, and a new raerismatic layer is 

 developed in the pericycle ; thus in Cycas, Encephalartos, and 

 species of Gnetum, the merismatic layer resembles the true 

 cambium in that it forms wood internally and bast externally, 

 and in these plants successive merismatic layers are formed ; 

 in Welwitschia the merismatic layer forms vascular bundles 

 and ground-tissue internally, and cortex externally, and persists 

 throughout the life of the plant. In the Cycadaceae and Coniferee, 

 the secondary wood consists exclusively of tracheides with rounded 

 or elongated bordered pits (scalariform tracheides) and of paren- 

 chymatous medullary rays, but true vessels are formed in the 

 Gnetaceoe ; the secondary bast has genei^lly the normal structure, 

 but in some cases (Abietineae) 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. Cunningharaia, 

 Abies, Taxus, Ginkgo, etc.), or on both sides (some Araucarias, 

 Podocarpus), or on the upper side only (Juniperus), but on all 

 sides when the leaf is acicular (e.g. Pinus, 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 : the acicular 

 leaves (Abietineee) have a single central vascular strand en- 

 closing two bundles which give off no branches ; in the flattened 

 leaves there may be several ribs which either do (e.g. Ginkgo) or do 

 not (e.g. Dammara, Araucaria) branch in the lamina, and in all 

 these cases the bundles end blindly ; in Gnetum the leaf, and in 

 Stangeria the leaf-pinna, has a midrib and pinnate venation; the 

 multicostate leaf of Welwitschia has parallel venation. A remark- 

 able feature in the structure of the leaf is the presence, in all the 

 genera, of a tissue, termed transfusion-tissue (p. 169), which consists 

 of parenchymatous cells, some of which contain no protoplasm and 

 have pitted walls, being in fact tracheides, whilst others con- 

 tain protoplasm and have unpitted walls : in the Abietineoe this 

 tissue is a development of the pericycle of the vascular strand, 



