SELAGINELLACE^ 45 



gether. The chlorophyll, both in the epidermal cells and in the meso- 

 phvll, is collected into large lumps, in which are grains of starch. 

 Stomates occur in the under, rarely also in the upper surface. The 

 mesophyll consists of a loose spongy parenchyme ; when the leaves are 

 very small it is developed only as a single layer surrounding the central 

 ' vascular ' bundle, and is altogether suppressed near the margins, where 

 the upper and lower epiderm are in actual contact. 



True roots occur in all known species belonging to the order. In 

 some species of Selaginella a structure known as the rhizophore inter- 

 venes between the stem and the root. The rhizophores resemble roots 

 in general appearance, but are destitute of a root-cap. They may spring 

 either from the dorsal side of the stem only, near the base of a branch, 

 bend round and then grow downwards, or two may spring from each 

 fork, one on the dorsal, the other on the ventral side, the former of which 

 usually remains undeveloped in the form of a small protuberance, while 

 the latter grows to the normal size. Their origin is very near the grow- 

 ing point, and they appear to be formed in the same way as the branches. 

 Unlike the roots, they are exogenous structures. After apical growth 

 has ceased, the end of the rhizophore, which is still very short, swells up 

 into a spherical form : its cell-walls become thicker, and the first rudi- 

 ments of the true root originate in the interior of the swelling, but do 

 not break through the surface until the rhizophore has increased consi- 

 derably in length by intercalary growth, and its swollen end has penetrated 

 the soil, where its apical cells deliquesce into mucilage, through which 

 the true roots reach the ground. In some species the rhizophores are 

 frequently transformed into leafy shoots, which at first manifest some 

 deviations from the normal structure of aerial shoots, but afterwards 

 present the ordinary structure, and may even bear sporanges. The 

 rhizophore is not, however, universal in Selaginella. In many species 

 the roots spring directly from the lowest fork of the stem, and branch 

 monopodially before they reach the ground. They originate, like those 

 borne by the rhizophores, near the growing point. All the roots branch 

 copiously, the planes of the successive branchings crossing one another 

 at right angles. They have a single apical cell, but this soon ceases to 

 give off segments, and the subsequent increase in length is chiefly due 

 to intercalary growth. 



Both kinds of sporange are shortly stalked nearly spherical capsules 

 (fig. 22), closely resembling those of Lycopodium in appearance and 

 structure, except in their being heterosporous. They are collected into 

 dense spikes at the extremity of somewhat metamorphosed leafy shoots. 

 The lower portion of each spike in some species consists of megaspo- 

 ranges, the upper portion of microsporanges, and the former may be 



