372 H01?MEISTER, ON THK HIGHER CRYPTOGAMIA. 



which exhibit a considerable supplementary longitudinal 

 development of the internodes, there is found the most 

 decided tendency to form an axile woody body, similar to that 

 whose production in Isoetes might — by the omission to 

 notice the analogous phenomena in the stem of Lycopodium 

 — be attempted to be brought into causal connexion with 

 the complete suppression of an intercalary multiplication of 

 the joints of the stem. 



As far as our knowledge extends, Isoetes alone of all the 

 vascular cryptogams possesses a cambium layer, which is 

 renewed yearly, and a stem which grows in length both at 

 the upper and lower ends ; peculiarities of which the one is 

 rendered necessary by the existence of the other. By the 

 organization of its stem, especially that of the downward- 

 growing portion of the w T ood, Isoetes approaches nearer to 

 Dicotyledons, — such as Cyclamen and Beta, — which have 

 undeveloped stem-joints and a stem which does not die from 

 below, than to the few Monocotyledons with diametrically- 

 increasing stems, such as Dracaena, Cordyline, and Tamus. 

 The mode of arrangement of the roots which spring from 

 the lower portion of the wood of Isoetes answers to that of 

 the adventitious roots which break forth in vertical rows 

 from the main root of dicotyledons, a phenomenon which 

 Schimper and Sachs* have stated and proved to be of uni- 

 versal occurrence. Some of the commonest plants in culti- 

 vation, such as turnips and radishes, exhibit such a manifest 

 regularity in the position and mode of succession of the 

 adventitious roots, that more certain general results may in 

 all probability be attained. 



* ' Sitzungsber. "Wiener Akad.,' B. xxvi (1858), p. 331. 



