176 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



closely related and are evidently determined by the same 

 conditions, viz., age, zone, and locality. It would be diffi- 

 cult to say which of these three agencies exerts the greatest 

 influence on the plants. All seem to be of equal importance. 

 A very good illustration is yielded by P. perforata. While 

 young specimens of P . perforata (three to four centimeters 

 long) are usually irregularly expanded, with a tendency 

 towards the orbicular, we notice that very soon a change in 

 the shape of the frond takes place. As a rule, specimens 

 five centimeters or more in length already show the type, 

 lanceolate with undulate margin; but older specimens, and 

 this applies particularly to those found in the lower part of 

 the litoral zone, and in the upper part of the sublitoral 

 zone, possess a great width and are frequently much lobed 

 and laciniate. Especially those plants which grow on the flat 

 surfaces of rocks, for instance on reefs, show a marked iso- 

 diametric development. But if the plant grows pendant from 

 an overhanging rock, it develops the elongated type of frond. 

 Another condition, and dependent upon locality, is the 

 movement of the water. Plants growing where they are 

 continually exposed to the wash of the waves, back and 

 forth, and from side to side, show far less marked longi- 

 tudinal development than those which are exposed to the 

 movement of the water in but one direction. This is very 

 well illustrated by P. naiadum And. f. major. The author 

 had an opportunity to observe this plant growing on Zostera 

 in the lagoon at Bolinas, Marin County, California. The 

 lagoon, which is long and narrow, was, in the summer of 

 1899, protected b}^ a high bar, so that at the rise of the tide 

 the water flowed in very regularly for a number of hours, 

 till the turn of the tide, when it flowed out as regularly. 

 The blades of Zostera and the fronds of P. naiadum were 

 bent in the direction taken by the water and the latter showed 

 a marked elongation, so great, indeed, that the writer felt 

 entitled to consider them a special form of the species, 

 since they were fully twice as long as the blades of P. 

 naiadum growing on Phyllospadix , and since there existed 

 some other minor differences as well. The latter form was 

 consequently designated as P. naiadum i. minor. 



