36.2 HOFMEISTER, ON 



and two-year old germ-plant : the two-edged lower end of its 

 upper part retains its form unchanged during the entire life of 

 the plant (PI. LI, fig. 1; PL LII, fig, 1). The formation of 

 new wood around that already present seems only to last 

 during a few vegetative cycles. All longitudinal sections of 

 plants from three to eight years old exhibit a somewhat 

 exuberant enlarged growth of the wood close under the 

 upper end. This locus of the greatest thickness of the wood 

 consequently moves continually upwards during the develop- 

 ment of the plant. 



The primary portions of the vascular bundles which 

 passed off into the leaves and roots formed many years pre- 

 viously, and which portions are attached to the wood, are 

 compressed by the cambium surrounding their sides, which 

 is always in a state of active vitality. Ultimately these 

 portions are torn off and pushed outwards, and the stump 

 which adjoins the mass of the wood is grown over by the 

 cambium just in the same manner as the stem of a tree gets 

 rid of the boughs of its lower portion. 



The vigorous leaves of plants of many years' growth ex- 

 hibit in their earliest stages, when viewed in front, the 

 ladder-like arrangement of their cells (PI. LII, fig. 7) which 

 is the necessary result of the mode of multiplication of the 

 cell of the first degree. This however soon becomes indis- 

 tinct by the rapid and vigorous development of the leaf in 

 thickness. The form, when viewed from above, of leaves 

 which are somewhat more developed (PI. LI, fig. 5) leads to 

 the conclusion that now, after each two divisions by septa 

 at right angles to the fore and hind surfaces of the leaf, septa 

 are formed in the terminal cell at right angles to the lateral 

 surfaces of the leaf and turned towards its front or hind side. 



Isoetes lacustris exhibits a manifest periodicity in the 

 interchange of sterile and fertile leaves.* In the terrestrial 

 species this interchange is very striking. Microscopical 

 investigation shows that the rudiments of the leaves are 

 formed a full year before they are developed ; the fruit- 

 bearing ones being produced late in summer and in autumn, 

 and the sterile ones in spring and early summer. During 



* Described by Alexander Braun in the 'Mora,' 1847, p. 34. But see 

 Bischoff, ' Krypt. Gewachse,' p. 84. 



