CARBONIFEROUS LYCOPODS AXD SPHKNOPHYLLS. 27 



As far as we are concerned at present it will be sufficient for 

 our purpose to consider shortly the three following genera — of 

 Section A, Lycopodium; and of Section B, Selaginella and Isoetes 

 — as it is amongst these that we will find the nearest allies to 

 our Carboniferous Ljcopods. 

 A. Lycopodie.e. 



Lycopodium, Linn. 

 The centre of the stem is occupied by a cauline central vascular 

 cylinder. This usually consists of several parallel, transverse 

 bands of xylem, composed of scalariform tracheides pointed at 

 both ends. These bands anastomise at intervals. If such a band 

 were dissected out it would appear as a somewhat irregular mesh- 

 work. Though the bands of the fibro-vascular portion of the 

 bundle thus form a united whole, each of the bands composing 

 the united structure may be regarded as an individual bundle. 



The central portion of the fibro-vascular bands consists of large 

 scalariform vessels ; at the two extremities of the band are much 

 smaller spiral vessels. Lying between and surrounding these 

 fibro-vascular tracts is the phloem, in which the sieve-tubes lie in 

 lines, and are distinguished by their large size. Lying outside 

 the phloem are some layers of broader cells — the phloem-sheath 

 of Hagelmaier, or pericycle, the whole being surrounded by the 

 bundle-sheath, or endodermis. 



Outside the phloem-sheath lies the cortex, the innermost layer 

 of which is parenchymatous or sclerenchymatous, but not coloured 

 as in the ferns. Surrounding this is a less indurated layer of 

 tissue, the whole being enclosed by the epidermis. 



Such is the general structure of the stem, but it varies in some 

 minor details in the different species. 



In some Lycopods the branching is monopodial, in others it 

 is dichotomous. The leaves are small, single-nerved, and 

 generally spirally placed on the stem. 



The sporangia are borne on the base of unaltered leaves, as in 

 Lycopodium Selago, Linn., or on the base of bracts or sporophylls 

 aggregated into distinct terminal cones, as in Lycopodium, 

 alpinum, Linn. The sporangia are reniform, and placed trans- 

 versely on the leaf or bract, to which they are attached by a 

 broad short stalk. They open by a cleft which runs across the 

 apex in their longer direction. The wall of the mature sporangium 



