4o8 



VA SC ULA R CR YP TO GA MS. 



internodes. In Selaginella the end of the stem rises above the youngest leaves as a 

 slender cone ; in Lycopodium it is blunt and flat. The branches of the dichotomies 

 grow with equal vigour in Psilotum, and often also in Lycopodium ; but in the latter 

 genus and in Selaginella some of the branches develope into primary stems or 

 branches, v/hich either assume a creeping position as rhizomes or an ascending one 

 as aerial stems. In Selaginella a tendency prevails to sympodial scorpioid develop- 

 ment of the dichotomous systems of branches (see p. 157) which not unfrequently 

 leads to the system of abundantly branched shoots developed bilaterally in one plane 

 attaining a definite outline, and a corresponding resemblance to a compoundly 

 pinnate leaf. The small size of the leaves in these genera causes the general habit 



to be mainly dependent on the development 

 of the systems of branches. 



The Leaves of Lycopodiacese are always 

 simple, unbranched, penetrated by only a 

 single fibro-vascular bundle, terminating in a 

 simple point, and ending, in Selaginella and 

 Lycopodium, in a fine awn. The largest 

 leaves occur in Isoetes, where they attain a 

 length of from 4 to 60 cm. They are in this 

 case divided into a basal part or sheath, and 

 an upper part or lamina. The sheath does 

 not entirely embrace the stem, but rises 

 in a somewhat triangular form from a very 

 broad insertion, and is acuminate ; it is 

 convex behind and concave in front, where 

 there is a large depression, the Fovea, con- 

 taining the sporangium ; the margin of this 

 depression rises in the form of a thin mem- 

 branous outgrowth, which, in many species 

 lies above the sporangium and envelopes it, 

 the Velum. Above the fovea and separated 

 from it by a 'saddle,' lies a smaller depres- 

 sion, the Foveola, the lower margin of which 

 forms a lip, the Labium, while from its bot- 

 tom an apiculate membranous structure, the 

 Ligule, with a cordate base, is prolonged 

 beyond the foveola (Fig. 306, A). The lamina of the leaf, containing chlorophyll, into 

 which the sheath passes above, is narrow and thick, almost cylindrical, but flattened 

 in front, and penetrated by four wide air-canals, which are divided by septa. This 

 form is exhibited by the fertile leaves of all the species of Isoetes ; a rosette of 

 such leaves is produced annually ; but between each pair of annual whorls is 

 formed a whorl of imperfect leaves, which consist, in /. lacustris, of only a small 

 lamina, but in the terrestrial species are destitute even of this, and may therefore 

 be considered as scale-like hypsophyllary leaves (phyllodes). 



The leaves of Sekginella are never more than a few millimetres in length, and 

 are usually cordate at the base with a narrow insertion, acuminate, and from lanceo- 



Fig. 306. — A longitudinal section through the base 

 of a leaf of Isoetes lacustris with its microsporangium 

 miitWX unripe ; B longitudinal section of the lower part 

 of a young sporangium (X300) (after Hofmeister). 



