364 DR. SARAH M. BAKER AND MISS M. H. BOHLING ON 
plants of attached Pelvetia are therefore exposed to similar conditions of 
desiccation during пеар tides to those they would experience on a rocky 
shore. But, immediately above the Pelvetia zone, there is a belt of Suæda 
fruticosa bushes. These are sufficiently high (about 6 or 8 inches clear 
of the ground) not to interfere with the movement of air around the alge ; 
but, being on the south-easterly side of the plants, they afford a deep shade, 
which covers many of them throughout practically the whole day. The 
shade from these Suwda bushes is certainly deeper than that thrown by the 
close ranks of Aster or Salicornia on the salt marshes. In spite of this 
shadow, all the Pelvetia plants fruit freely and abundantly, and the 
receptacles produced are perfectly normal. No essential difference could 
be found between plants grown under (the shade of the bushes and those 
which were exposed to full sunlight. 
From this we concluded that the humidity of the salt marsh in the inter- 
tidal periods, due partly to the retention of water by the flat, undrained soil, 
and partly to the transpiration of the halophytes, is the primary cause of the 
sterility of marsh Fucoids. 
This hypothesis agrees well with other observations. The only two species 
which produce an abundance of normal fruit-bodies on the salt marsh, 
namely, Fucus spiralis у. nana and Ascophyllum nodosum ecad Маскай, both 
grow in comparatively exposed positions, where the humidity might become 
low in the intertidal periods. F. spiralis v. nana hangs on vertical or sloping 
mud-banks, usually destitute of halophytic vegetation ; while A. nodosum 
ecad Л/аскай is described as lying in masses on mud, sand, or pebbles 
among boulders upon the seashore, in situations certainly less exposed but 
quite analogous to the habitat of the ordinary attached plant. 
Further evidence is afforded by the marsh forms of Fucus ceranoides 
found at Keyhaven, Hants. Here large, apparently normal, plants grew on 
the marsh, aitached to the rhizomes of rushes, as well as the unattached 
dwarf marsh form; yet these attached plants, evidently derived from 
fertilized oospheres, showed exactly the same abortion of the sexual organs 
as the dwarf marsh plants. This derangement of the sexual organs affected 
them in two ways. First, the oogonia were not divided ; secondly, the 
proportion of the sexes in a normally hermaphrodite conceptacle became 
variable, so that on the same plant receptacles of either sex might be 
produced, together with all intermediate stages between that and the 
hermaphrodite condition. 
These observations indicate that the humidity of the marsh habitat pre- 
vents a certain concentration of the cell-sap, requisite for the production of 
sexual organs. Probably this is of the nature of a stimulus, for we have not 
been able to trace any difference in the fertility of plants taken from the 
wide vertical range of Fucus vesiculosus on a rocky shore, nor is there any 
correlation between the time of liberation or production of gametes and the 
