408 



PART III. THE CLASSIFICATION OF PLANTS. 



sporangia, either microsporangia or macrosporaugia, but not both, 

 surrounded by an inferior indusium (Fig. 268). In Salvinia the 

 sori are borne at the apices of the basal branches of a submerged 

 leaf, and may be comparatively numerous (4-20) on one leaf : in 

 Azolla the sori are borne at the apices of the segments of the lower 

 (ventral) submerged lobe of a leaf, and that leaf is always the 

 first (basal) leaf of a fertile branch which is sometimes less 



vigorously developed than the 

 purely vegetative branches; each 

 leaf usually bears only two sori, 

 but in A. nilotica there are four. 

 In Salvinia the sori are all of the 

 same size, whereas in Azolla the 

 sori containing microsporangia are 

 considerably larger than those con- 

 taining macrosporangia. In both 

 genera the tip of the fertile leaf- 

 segment expands into a cellular 

 cushion, the placenta, from the 

 superficial cells of which the spor- 

 angia are developed : the indusium 

 is developed as an annular out- 

 growth from the base of the pla- 

 centa, becoming cup-shaped, and 

 eventually closing over the sorus: 

 it consists of two layers of cells 

 which, in Salvinia, are separated 

 by large air-chambers and are con- 

 nected by longitudinal cellular 

 trabeculae. In both genera the 

 microsporangia of a sorus are nu- 

 merous (about 40 in Azolla, more 

 in Salvinia) : the macrosporangial 

 sorus consists, in Salvinia, of many 

 (up to 25) macrosporangia, whereas 

 in Azolla there is but one. Both 

 kinds of sporangia are borne by 

 the same plant. 



In the Marsileaceae the sporo- 

 carp consists of a leaf-branch enclosing a number of sori, and 

 each sorus includes both micro- and macrosporangia. In Marsilea 



FIG. 263.^ Apical portion of the 

 stem of Salvinia natans, seen obliquely 

 from below (nat. size) : 1 1 aerial leaves ; 

 loio aquatic leaves, with sori, ss; fc ter- 

 minal bud of the stem. B Longitudinal 

 section through three fertile teeth of 

 an aquatic leaf (x 10), forming two 

 sori with microsporangia, (u) one with 

 macrosporangia ; i indusium. (After 

 Sachs.) 



