28 TRANSACTIONS, NATURAL HISTORY SOCIKTY OF GLASGOW. 



is composed of two layers of cells. The spores are numerous, 

 very small, and of one kind, roundish or tetrahedral in form, 

 with various markings on the exosporium. 



For our present purpose it is not necessary to enter into 

 further details of the structure or development of Lycopodium. 



B. SELAGINELLEiE. 



Selaginella, Linn. 

 The stem contains one or several cauline vascular bundles, 

 which are ribbon-like in form. The xylem portion is chiefly com- 

 posed of scalariform tracheides, the smaller and primary elements 

 being at the ends, which are annular or spiral. The xylem is 

 entirely surrounded by the phloem, and the stem vascular 

 bundles are very similar in structure to those of ferns. The 

 phloem is enclosed by two or three laj^ers of parenchyma, which 

 may be regarded as the pericycle. Surrounding these is a large 

 air-space, " which is only interrupted by transverse cellular 

 filaments, like flying buttresses, to support the bundles." These 

 probably represent the endodermis. 



The tissue surrounding the air-space and its contained bundle 



consists of thin-walled elongated cells, the whole stem being 



encased by an epidermis of long 



4 il , prosenchymatous tissue. The branch- 



ti....,\ 1 ( h ing is lateral monopodial. 



\^ (^_J y^ On the typical Selaginella the leaves 



^, \\ I are generally of two kinds — "the two 



B rows of the lower plane are more 



^_^^,^j=_:<»_si..j;^^ a spreading; the two rows of the upper 



^r''^ )<-_ ascending, adpressed to the stem and 



N^j,^ y imbricate." ^ On the upper side and 



^^^^'^-s-j^-TT-^ near the base each leaf bears a ligule. 



A (Fig. 1.) 



Fig. 1. Se/agineUa sp. A, The fruit is borne in the form o£ 



Leaf showing position of ligule. terminal cones. The lower bracts, or 



B, Base of leaf with ligule a. sporophylls. usually bear macrospor- 



Both figures enlarged. ^,^^^^^ ^j^^ ^pp^^, microKporangia; but 



occasionally the macrosporangia and microsporangia may be side 



by side with each other. 



* Baker, Handbook to the Fern Allies, p. 31. 1S87. London, 



