28 VASCULAR CRYPTOGAMS 



again divides by a septum nearly at right angles to the first one. Further 

 divisions then take place. Out of the posterior of the first two segments 

 (the one immediately beneath the mouth of the archegone) is formed 

 the foot of the young plant, by which it is attached to and derives its 

 nutriment from the prothallium. From the anterior of these two seg- 

 ments is derived a peculiar foliar structure, differing from all the subse- 

 quent leaves, the cotyledon or scutiform leaf, by the growth of which the 

 terminal bud of the stem becomes directed downwards. No root what- 

 ever is produced. Azolla is stated to have a second cotyledon. Both 

 stem and root (in Azolla) are developed from a single apical cell, which 

 is rounded in front and pointed below. The primary meristem-layers 

 are differentiated, as in Flowering Plants, into plerome, periblem, and 

 dermatogen. 



The mature sporophyte differs considerably in appearance in the two 

 genera, but always floats on the surface of the water. The very short 

 stem is erect or horizontal, and the branching of both stem and root (in 

 Azolla) is monopodial. The root, stem, and leaf-stalk are each traversed 

 by a single concentric ' vascular ' bundle of simple structure, containing 

 spiral and annular tracheides. The leaves of Azolla are very crowded, 

 and are placed in two rows on the dorsal side of the stem ; but in some 

 species they have the appearance of standing in four rows. They are of 

 delicate membranous texture, and are always deeply bifid, one lobe being 

 submerged and the other floating. The floating lobe of each' leaf has a 

 remarkable cavity, covered by a double epidermal layer, with the excep- 

 tion of a narrow orifice which opens into the cavity. This cavity is 

 formed during the growth of the leaf by a more rapid growth of the 

 epiderm than of the subjacent tissue, and is itself clothed with an epi- 

 dermal layer. The cavities are frequently occupied by well-developed 

 colonies of Nostoc-filaments. Salvinia is remarkably heterophyllous 

 (see figs. 7, 8). The first leaf of the young plant is the scutiform or 

 peltate leaf already mentioned, which is produced near the base of the 

 stem. It is coriaceous in texture and sagittate in form. Next are pro- 

 duced, also singly, two ovate aerial leaves. All the subsequent leaves 

 are arranged in whorls of three, two of which are aerial, with flat, ovate, 

 or orbicular lamina, floating on the surface of the water ; while the third 

 or submerged leaf at once branches into long slender filiform segments, 

 which hang down into the water and perform the function of roots. The 

 leaves of adjacent whorls are placed alternately, so that the mature plant 

 has two rows of ventral submerged, and four rows of dorsal aerial 

 leaves. Each leaf has a single definite apical cell in Salvinia, but not in 

 Azolla. The leaves of both genera are furnished with stomates, which, 

 according to Strasburger, differ considerably, both in structure and 



