GNETACE.E. ,5^ 



of the wood, and in the primary and secondary cortex, as well as distributed through 

 the leaves (p. 105) ; always following the direction in length of the organs, like the gum- 

 passages of Cycadea^. In many Conifers with short leaves roundish resin-glands also 

 occur in them (as in Callitris, Thuja, and Gupressus, according to Thomas); in Taxus 

 the resin-canals are entirely wanting ^ 



The Lea'ves of Cycadeae and Goniferae are covered by a firm epidermis, usually 

 strongly cuticularised, and furnished with numerous stomata, each with two guard-cells. 

 In the Gycadeae the guard-cells are more or less deeply depressed, and the stomata occur 

 only on the under side of the lamina, and are either irregularly scattered, or arranged in 

 rows between the veins (Kraus). In the leaves of Conifers the guard-cells are also, 

 according to Hildebrand (Bot. Zeit. 1869, p. 149), always depressed in the epidermis; 

 and the stoma has hence always a border (r/. p. 86). In Goniferae the stomata are 

 developed either on both or only on one side of the leaf; when the leaf is broad, 

 as in Dammara and Salisburia, they are irregularly scattered; when the leaves are 

 acicular they mostly lie in longitudinal rows ; and in the large leaves of Welwitschia 

 they are also arranged in rows. The firm texture of the leaves of Cycadeae and Goni- 

 ferie is due to a hypodermal layer (p. 105), often strongly developed, consisting of 



Fir.. 336. — Pinus Ptiiaslry : two cells of the colourless parenchyma surroiincling the fihro-vascular bundle 

 of the leaf; r/ the ilot-like structures. cut across, t' the same seen from the surface. 



strongly-thickened, generally long, fibre-like cells lying parallel to the surface; in the 

 leaf of Welwitschia this hypoderma consists of spongy succulent tissue penetrated by 

 bundles of fibres, which acquires its hardness from a mass of spicular cells. The 

 chlorophyll-tissue of the leaves lies beneath this layer, and is developed on the upper 

 side of the leaves of Cycadeae and of the broader leaves of Goniferae as the so-called 

 Pallisade-tissue ; /. e. its cells are elongated in a direction vertical to the surface of 

 the leaf and are densely packed together. In Pinus, Larix, and Gedrus the cells which 

 contain chlorophyll exhibit the infoldings of the cell-wall which have been already 



* [Van Tieghem (Ann. des Sci. Nat. 1872) distinguishes the six following modifications of the 

 distribution of the secretory organs in Coniferce :— i. No canals in the root nor stem : Taxus. 2. No 

 canals in the root ; canals in the cortical parenchyma of the stem : Cryptomeria, Taxodium, Podo- 

 carpus, Dacrydium, Torreya, Tsuga, Cunninghamia. 3. No canals in the root; canals in the 

 cortical parenchyma and in the pith of the stem : Salisburia. 4. A secretory canal in the root ; 

 canals in the cortical parenchyma of the stem : Gedrus, Abies, Pseudolarix. 5- Canals in the wood 

 of the fibro-vascular bundles of the root and stem ; canals in the cortical parenchyma of the stem : 

 Pinus, Larix, Picea, Pseudotsuga. 6. Canals in the liber of the fibro-vascular bundles of the root 

 and of the stem; canals in the cortical parenchyma of the stem: Araucaria, Widdrmgtonia, Thuja, 

 Gupressus, Biota. In Cycade^ the canals are found disseminated through the cortical parenchyma 

 of the stem ; the pith of Cycas appears destitute of them. In their distribution they resemble 

 therefore that which occurs in the second class of Conifer^.— Ed.] 



Hh 



