The Gemis Phyllospadix 415 



the later type. Second, the conditions in which the ancestors 

 of Phyllospadix, on this coast, found themselves, have forced, 

 in the growth of the genus, the development of an unusually 

 strong stereome, or system of strengthening cells in stem and 

 leaf, to which Zostera has no tissue which will bear any com- 

 parison, and a symmetrically balanced rhizoma with a remark- 

 able system of shields and sheathing members, designed to 

 protect the buds and young reproductive organs. 



In the morphology and anatomy of a plant, I believe we 

 may find at least a partial record of the influences of past 

 environment in the struggle of the organism, not only for 

 existence, but for upward development ;— /. ,?., for a mode of 

 living which will make the best use of the resources at its 

 command. In its present environment, if likely to have been 

 long continued, we may find still further explanation and cor- 

 roboration of the structural evidence. In looking for the 

 causes which lead to the evolution of a new genus we may 

 profitably consider a similar line of evidence, adding the testi- 

 mony to be derived from geographical changes and geological 

 records. 



We have already sufficiently explained the present habitat 

 of Phyllospadix, on stony and rocky shores, and connected its 

 peculiar structure with the supposed effort on the part of the 

 organism to meet the conditions (for a plant) of an unusually 

 stirring existence. But why a branch of the shallow-water 

 Zosteras should have ventured on this bold career, and what 

 the conditions really were during its earlier years, cannot per- 

 haps be more than imperfectly answered, in the present state 

 of our knowledge. 



Referring again to the distribution of living sea-grasses, 

 and leaving out of account the two Zosteras of wide distribu- 

 tion, and the few doubtful species and stations, we find there 

 are certain geographical centers of development. And we 

 may suppose these regions to have preserved something in 

 their conditions, at least not unfavorable to the continuance, 

 and even favorable to the further development of the old ge- 

 ologic types. They are grouped as follows : 



