W. HOFMEISTER ON THE DEVELOPMENT OP ZOSTERA. 259 



opposite to it. The Ferns and Rhizocarpeae belong to the first 

 of these sections, Selaginella and Isoetes to the second. The 

 peculiar arrangement of the roots of Isoetes does not follow so 

 much from the condition just indicated, as from the phenomena 

 of the growth of the mantle of cambium, annually renewing the 

 cortical tissue, surrounding the wood-structure ; combined with 

 the suppression of the internodes. In Zostera, Ruppia, and in 

 the Grasses, the mode of appearance of the first adventitious 

 root is the same as in Isoetes. It springs out opposite the first 

 leaf, not behind it. The great majority of the Monocotyledons 

 seem to behave in the same way. In Lemna, on the contrary, 

 the position of the first adventitious root corresponds to that of 

 the Ferns. To carry through the comparison of the two classes 

 in this respect will require still more comprehensive researches. 



EXPLANATION OF PLATES VI, VII and VIII. 



Fig. 1. Longitudinal section of the terminal bud of a fertile plant of Zostera 

 minor, magnified 30 diam. : a the third, b the second, c the first 

 (youngest) spadix ; a 1 , 6 1 , c l , the sheathing basal leaves (niederbldtter) 

 of the sprouts terminating as spadices; a 2 , 6 2 , c 2 , the stem-leaves 

 (laub-blatter) of the same ; d, the new sprout arising in the axil of 

 the sheathing leaf of the youngest sprout. 



Fig. 2. A similar preparation of Zostera minor. The section, not carried 

 quite accurately through the slit of the sheathing leaf of the sprout of 

 the penultimate rank, has hit the overspreading part of this sheath. 



Fig. 3. Apex of a spadix of Z. marina commencing a longitudinal develop- 

 ment, seen from above. Magnified 40 diam. : a, anthers ; b, ovaries. 



Fig. 4. Young anther (a) and rudiment of an ovary (b) of Zostera minor, seen 

 from above. Magnified 120 diam. 



Fig. 5. Transverse section of a very young half-anther of Z. minor. Only the 

 contents of the primary parent-cell of the pollen represented. Magn. 

 350 diam. 



Fig. 6. Similar preparation from a somewhat more developed anther. Magn. 

 350 diam. 



Fig. 7. Some of the primary parent- cells of the pollen, from the longitudinal 

 section of a very young half-anther of Z. marina. Magn. 400 diam. 



Fig. 8. Lower portion of a longitudinal section of a half-anther of Z. marina, 

 parallel to the surface of the spadix ; a, cells of the septum between 

 the two loculi. Some of the elongated parent-cells of the pollen 

 cut across. Magn. 400 diam. 



Fig. 9. Portion of the longitudinal section of a half-anther of Z. marina imme- 

 diately after the formation of the pollen-cells. Magn. 400 diam. 



17* 



