CHAPTER XII. 



ISOETES LACUSTUIS. 



The development of the Isoeteae is a subject of great 

 importance in botanical morphology. They are the only 

 known family in which the principal axis never ramifies. 

 As far as present observations extend, they alone, in the 

 vegetable kingdom, are distinguished by the entire sup- 

 pression of a supplementary cell-multiplication in the joints 

 of the stem. In other stems, however little their inter- 

 nodes may be developed, an active multiplication of the 

 cells in a longitudinal direction takes place (after the for- 

 mation of the youngest internode) in the second youngest, 

 or even in the adjoining internodes. In Isoetes, after the 

 formation of one internode, the longitudinal growth ter- 

 minates absolutely. The features by which the Isoeteae 

 are clearly distinguishable from the plants nearest allied to 

 them in their mode of reproduction are as follows : — the 

 development of adventitious roots (apparently in a descend- 

 ing series), the form of the ligneous mass, and especially 

 the existence of a mantle of cambium surrounding the 

 wood and retaining its activity during the whole life of the 

 plant. The processes of impregnation are seen in Isoetes 

 with greater facility and clearness than in any other cliae- 

 cious cryptogams. 



Hugo von Mohl pointed out the peculiar phenomena of 

 growth of the Isoetege,* viz., the development of the adven- 

 titious roots in an apparently descending order on both-* 

 ■sides of a furrow traversing the under surface of the 



* 'Linnsea/ 1840. c Vermischte Scliriften/ p. 122. 



