28 BULLETIN 1191, U. S. DEPARTMENT OF AGRICULTURE. 
SALINITY AND RAINFALL. 
Rains probably affect the growth of the plant but little. Coasts 
inhabited by it vary in rainfall from 10 to 60 or more inches per 
year. (See McFarland (5),and Bartholomew (7).) The author has 
seen fairly healthy plants of Nereocystis growing in the brackish 
estuary of Big River at Mendocino City; and it is a well-known fact 
that numbers of sea-weeds grow on the steamboats plying between 
Stockton and San Francisco, in spite of their being subjected to daily 
changes from fresh river water to salt sea water and vice versa. 
(Osterhout (6), pp. 227-2380.) 
LIGHT. 
The kelp plant contains chlorophy! and manufactures starch from 
CO,. It must, therefore, like green land plants, have light. No 
kelp is found in the dense shade under the oil docks on San Pedro 
breakwater, although there is an abundant growth at both sides of 
them and between them. It also seems to make a poor growth where 
it is shaded by a high bluff. On the other hand, just as in the case 
of land plants, very strong light appears to retard its growth. Plants 
near shore almost invariably have shorter and fewer fronds than 
plants growing in deeper water; and the fronds of shallow-water 
plants seldom float on the surface for more than 15 feet, whereas 50 
feet is not an unusual length for the floating portions of plants 
growing in water of a depth of 40 to 60 feet. Light may also have 
much to do with the reduction of rate of growth, and finally the 
complete cessation of growth in a frond after it has attained the sur- 
face of the water. 
SEASONAL VARIATION IN CONDITION OF KELP BEDS. 
In summarizing the subject of growth, owing to the influence of 
the various factors enumerated above, Macrocystis is restricted in its 
distribution to more or less widely separated groves along the sea 
coast, plants within the groves vary greatly in size and vigor, and 
the groves themselves vary greatly from year to year in density and 
in area. There is, moreover, a comparatively regular seasonal varia- 
i tion in the amount of kelp floating on the surface. July and August 
| may be called the dormant season, since few new fronds are coming 
H 
| 
H| up at that time and the old ones are rapidly decaying and disappear- 
h ing. In September, or early October, activity is resumed and if the 
) water is cool, and storms are not too frequent, December sees a heavy 
mat of kelp on the surface. The frequent windstorms of January, 
February, and March cause rapid wearing away of the leaves and 
destroy many fronds and even entire plants, so that although the 
plants are growing with maximum rapidity at that season, the 
quantity of floating kelp is somewhat reduced. In April, with the 
| continued rapid growth of the plants and the less frequent occurrence 
| of storms, the density of the floating mat is soon restored, and this 
heavy mat persists until June, when, with the warmth of the water 
| increasing, decay begins to gain on regeneration. In the tables, the 
| yield of kelp per unit of area is taken as an index of the rate of 
i growth. Such a conclusion may seem unfair, since the entire area of 
