ULVACE^ — THEIR MODE OF DEVELOPMENT. 



309 



Of the third condition, we have an example in the eurions Pal- 

 modictyon described by Kutzing; the frond of which appears to 

 the naked eye Uke a delicate network consisting of anastomosing 

 branches, each composed of a single or double row of large vesi- 

 cles, within every one of which is produced a pair of elliptical 

 cellules that ultimately escape as "zoospores." The alternation 

 between the " motile" form and the " still" or resting form, which 

 has been described as occurring in Protococcus (§§ 153-155), has 

 been observed in several other forms of this group ; and it seems 

 obviously intended, like the production of "zoospores," to secure 

 the dispersion of the plant, and to prevent it frota choking itself 

 by overgrowth in any one locality. No other form of the true 

 Generative process, than the "conjugation" of two cells, and the 

 formation of a spore by the reunion of their contents, has yet been 

 observed among the members of this family ; and as this has only 

 been witnessed in a few of them, it is probable that we do not yet 

 know by any means the whole of their life-history. And from 

 the close resemblance which many reputed Palmellacece bear to 

 the early stages of higher 

 Plants (Fig. 104, a, b, o), Fw- io4. 



there is considerable doubt 

 whether they ought to be 

 regarded in the light of dis- 

 tinct and complete organ- 

 isms, or whether they are 

 anything else than embryo- 

 nic forms of more elevated 

 types. Here, again, there- 

 fore, there is an ample field 

 for investigation to the Mi- 

 croscopist who desires to 

 employ himself in extending 

 the boundaries of Science, 

 and in perfecting our very 

 imperfect acquaintance with 

 thes* humble but most in- 

 teresting and instructive 

 types of Vegetation. 



195. I^Totwithstanding the 

 very definite form and large 

 size attained by the fronds 

 or leafy expansions of the 

 JJlvaeece, to which group be- 

 long the grass-green sea- 

 weeds (or "lavers") found on 



every coast, yet their eSSen- successive stages of development of Ulva. 



tial structure difters but very 



little from that of the preceding group ; and the principal 



advance is shown in this, that the cells, when multiplied by 



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