290 



THIRD GROUP. VASCULAR CRFPTOGAMS. 



outline, but flattened in front and traversed by four broad air-canals which are 

 segmented by transverse plates. This is the form of the fertile leaves of all the 

 Isoeteae, and a rosette of such leaves is formed every year ; but between every two 

 circles of these leaves there appears a circle of imperfect leaves, which in /. lacustris 

 have nothing but a small lamina, while in terrestrial species the lamina is so reduced 

 in size that the leaves are only scale-like arid cataphyllary. 



The leaves of the Selaginelleae are never more than a few millimetres in length ; 

 from a narrow insertion they are generally broadly cordate and acuminate above, but 

 may be ovate or lanceolate. In most species the sterile leaves are of two sizes ; those 

 on the under or shaded side of the obliquely ascending stem, the ventral leaves, are 

 much larger than the dorsal leaves on the upper or illuminated side of the stem 

 (Fig. 235 A). A ligule is also found on the upper side of the leaf above the base ; 

 the sporangium stands below the ligule on the fertile leaves, which form a quadran- 

 gular terminal spike, and are of uniform size and usually of a somewhat different 

 shape to the sterile foliage-leaves. 



The phyllotaxis. The rosettes in Isoetes are arranged in spirals with the diver- 

 gences f, 1%, -O S T , ^f ; the divergences become more complicated as the number 

 of the leaves formed each year increases. In the Selaginelleae with dorsal and ventral 

 leaves in four rows one dorsal and one ventral leaf form together a pair, the median 

 line of which however does not intersect that of the adjacent pair at a right angle but 

 obliquely; this is often easily perceptible on older shoots of S. Kraussiana. 



The apex of the stem has no apical cell in Isoetes, but is occupied by a group of 

 meristem-cells. The different species of Selaginella vary much in this respect and 



afford much instruction. -5*. spinu- 

 losa, S. arborescens, and some 

 others have the vegetative cone 

 of the Lycopodiaceae, that is they 

 have no apical cell ; S. serpens, 

 S. Martensii, S. horlcnsis, etc. 

 have the two-sided apical cell 

 described above (Fig. 234 A}', 

 in 6". Wallichii Strasburger found 

 the apex of the growing stem 

 occupied by two apical cells of 

 equal size and with 'the shape of 

 an elongated pointed wedge with 



FIG. 234. Apex of the stem of Selaginella Martensii. A longitudinal four SUrfaCCS. But TrCUb 

 section of the extremity of the stem with the rudiments of the youngest 



leaves. B apex of the stem seen from above. The segments are indicated that the arrangement of the Cells 

 by thicker lines, the segments themselves are marked with Roman numerals. 



at the apex varies not only in dif- 



ferent species but in the same species. For instance the apical cell of S. Martensii 

 may be two-sided, or a three-sided pyramidal cell. 



It follows from what has been said that the lateral branches are never placed in 

 the axil of a leaf, but above one of the ventral leaves. They do not proceed from a 

 single cell, but from a group of cells which bulges out beneath the apex of the 

 primary shoot. At first the shoot thus formed has no apical cell ; such a cell is 

 formed in S. Martensii^ but it has at first the shape of a four-sided wedge such as is 



