374 



PART IV. CLASSIFICATION. 



In the Salviniacese the so-called sporocarp is simply a sorus of 

 sporangia, either micros porangia or macrosporangia, but not both, 

 surrounded by an inferior indusium (Fig. 224). 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;i leaf of a fertile branch which is sometimes less vigorously 



developed than the purely vegeta- 

 tive branches ; each leaf usually 

 bears only two sori, but in A. nilo- 

 tica there are four. In Salvinia 

 the sori are all of the same size, 

 whereas in Azolla the sori contain- 

 ing microsporangia are consider- 

 ably larger than those containing 

 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 sporangia are de- 

 veloped : the indusium is developed 

 as an annular outgrowth from the 

 base of the placenta, becoming cup- 

 shaped, and eventually closing over 

 the sorus : it consists of two laj-ers 

 of cells which, in Salvinia, are 

 separated by large air-chambers 

 and are connected by longitudinal 

 cellular trabeculse. In both genera 

 the microsporangia of a sorus are 

 numerous (about 40 in Azolla, more 

 in Salvinia) : the macro-sporangial 

 sorus consists, in Salvinia, of many 

 (up to 25) macro-sporangia, whereas 

 in Azolla there is but one. Both 

 kinds of sporangia are borne by the 

 same plant. 



In the Marsileacese the sporocarp consists of a leaf-branch 

 enclosing a number of sori, and each sorus includes both micro- and 

 macrosporangia. In Marsilea the fertile leaf -branches spring from 



FIG. 224. A Apical portion of the stem 

 of Salvinia natans, seen obliquely from 

 below (nat. size) : 1 1 aerial leaves ; ic >c 

 aquatic leaves, with sori, *s; 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, (a) one with 

 macrosporangia; t indusium. (After 

 Sachs.) 



