414 MOSSES AND FERNS chap. 



in the same manner. The endodermal cells from which they 

 spring are distinguished from the others by their shorter and 

 broader form, and are very easily recognisable by this as well as 

 from their position. They form two vertical rows exactly opposite 

 the ends of the xylem plate, and the lateral roots therefore 

 are also strictly two-ranked. Narrow lacunae are formed in the 

 cortical tissue of the root, and the cells surrounding these are 

 connected by regular series of short outgrowths, which connect 

 them in a way that recalls very strongly the connecting tubes 

 between conjugating filaments of Spirogyra, and produce a 

 similar ladder-shaped appearance. 



The solid vascular cylinder of the young stem is later 

 usually replaced by a tubular one, but its structure is also 

 concentric, with phloem completely surrounding the xylem, and 

 has both an inner and outer endodermis. When the plants are 

 completely submerged the ground tissue is mainly parenchyma, 

 but in the terrestrial forms sclerenchyma may be developed in 

 the cortex of the stem and petiole. The latter is always 

 traversed by a single axial bundle, which in the lamina in 

 Marsilia divides repeatedly near the base of the wedge-shaped 

 leaflets into numerous dichotomous branches. 



Luerssen -^ mentions as special reproductive bodies, tubers 

 found in M. hirsuta. These are irregular side branches covered 

 with imperfectly-developed leaves, and with the cortical tissue 

 strongly developed and full of starch. These are supposed to 

 survive long periods of drought, and to germinate under favour- 

 able conditions. A condition somewhat analogous to this 

 appears in M. vestita (Fig. 206, A), but whether these short 

 lateral branches are of this nature was not investigated. 



T]ie Sporocarp '^ 



The development is much the same in the two genera, but 

 is most easily followed in the simple sporocarp of Pilularia. In 

 P. Americana, the young fruit begins to develop almost as soon 

 as the leaf can be recognised, and while it is still close to the 

 stem apex. Growth is stronger upon the back of the young 

 leaf, and it very early assumes the circinate form. Before this 



1 Luerssen (7), p. 601. 



2 Sachs, Text-book, 2nd English edition, p. 455 ; Goebel (6) ; Juranyi (2) ;. 

 Russow (i), Meunier (i). 



