THE PTERIDOPHYTA : LYCOPSIDA, ETC. 579 



Older branches turn up vertically. The stem is clothed with leaves which 

 are arranged in four rows, the leaves being in pairs, each pair consisting of a 

 large and a small leaf. Those springing from the lower surface are con- 

 siderably larger than those originating from the upper surface. In this way 

 all the leaves are exposed equally to the light, and the lower leaves are not 

 overshadowed by the upper ones. At the base of each young leaf is a small 

 membranous outgrowth called the ligule which soon withers away. This 

 structure is characteristic of this family of the Lycopodiales. As it is also 

 present in some of the fossil tree-like forms, it is considered as good evidence 

 of the close relationship between these fossil trees and the small present- 

 day species of the genus Selaginello, and it is concluded that during the 

 Carboniferous period the group was of far greater importance than it is at 

 the present time. 



Arising at the base of each branch is a root-like organ, which is called a 

 rhizophore. This structure resembles a root in being leafless, but it differs 

 in not possessing a root cap. In S. kraiissiana it only reaches a length of a 

 few millimetres when it loses its apical growth and produces two equal roots, 

 which arise endogenously from cortical cells, thus simulating dichotomy. 

 If the rhizophore subsequently elongates, as it may in damp air, its growth 

 is intercalary. The roots produced grow downwards and on entering the 

 soil again branch and develop root hairs. The leafy stem itself bears no 

 roots. In epiphytic species of Selaginella, which may reach a length of 

 many feet, climbing among branches, the rhizophores are very important in 

 carrying the root system into deep crevices where moisture is held. 



From the stem certain vertical branches grow up which dilTer in structure 

 from the ordinary stem, since on them are borne the reproductive organs. 

 As in Lycopodium these organs are called strobili and their leaves sporophylls. 

 The sporophylls, like the vegetative leaves, are borne in opposite pairs, but 

 all are the same size. In Selaginella, the sporophyll bears a single sporangium 

 in the axil between the leaf and the stem. These sporangia are of two kinds, 

 megasporangia and microsporangia. The former encloses a small number, 

 generally four, of large megaspores, while the latter bear a large number of 

 small microspores. Selaginella is therefore an example of an heterosporous 

 plant. In most species both types of sporangia are produced on the same 

 strobilus. In Selaginella kraiissiana the microsporangia are usually produced 

 in the axils of the upper sporophylls while the megasporangia are developed 

 in the axils of the lower sporophylls. 



Anatomy of the Stem 



In section the stem is dorsiventral, flattened above and rounded below, 

 with two lateral grooves (Fig. 589). There may be one or several steles in 

 the stem, according to the species, and in Selaginella kraiissiana there are 

 two steles, each of which has one or sometimes two protoxylem groups at 

 the ends of the oval mass of xylem, which is therefore exarch. There is a 

 certain amount of parenchyma between xylem and phloem. The phloem 



