130 MARINE ANGIOSPERMS [CH. 



bearing a thick matting of root hairs ; this tangle of roots fixes 

 the plant in the sand. The flowers are typically hydrophilous. 

 The filiform styles, which may be 26 mm. long, are receptive 

 throughout their entire length, and, though the individual pollen- 

 grains are not thread-like, the same result is secured by their 

 being united into strings 1 . The seed-coats form an admirable 

 protection for the embryo. The outermost cell-layer is conspi- 

 cuously thickened on all the walls except that forming the sur- 

 face of the testa. The next three cell-layers are cuticularised. 



FIG. 87. Halophila ovalis, (R.Br.) Hook. fil. Portion of mature plant showing two 

 female flowers in spathes with three thread-like stigmas (st.) . (Enlarged.) [Balfour, 



I. B. (1879).] 



Since the testa of Zostera is similar in structure, it seems not 

 unlikely that in both cases the histological features bear some 

 relation to the mode of life. Bayley Balfour concludes, from the 

 general result of his researches, that Halophila forms a link 

 between the Hydrocharitaceae and Potamogetonaceae. 



The leaf anatomy of the marine Helobieae has been studied 

 in great detail, partly because these plants are nearly always 



1 The thread-like character of the pollen of Halophila was observed by 

 Gaudichaud, C. (1826) who also noticed the same feature in Cymodocea 

 antarctica. 



