20 BULLETIN 1191, U. S. DEPARTMENT OF AGRICULTURE. 
grow at a time; and the third has creeping stems which may cover 
extensive areas and produce innumerable fronds. 
Plants often live 5 years or longer, and the creeping type may 
live an unlimited number of years. 
Plants attain a length of from 100 to 150 feet. 
GROWTH WITH REFERENCE TO ENVIRONMENT. 
The important factors influencing the growth of Macrocystis pyri- 
jera are substratum, depth of water, temperature, winds, waves, light, 
salinity, and rainfall. 
SUBSTRATUM. 
* The Laminariacez, like other fixed alge, require a firm foothold ” 
(Setchell, 7). Because of the great size and number of fronds spread 
out on the surface of the water, Macrocystis must be very firmly 
anchored or the winds and waves will bring it ashore. Many of the 
plants brought ashore are healthy growing plants which quite ap- 
parently have been torn loose from their anchorage, and it may safely 
be said that the kelp beds depend for their existence upon the pres- 
ence of ledges or reefs, or bowlders of sufficient weight or of sufficient 
depth in the mud to hold the plants in place during stormy weather. 
SEASONS 
Yield in hundreds of tons per square mile 
Ses 7 27 
Spring 
Summer 
Fig. 11.—Seasonal variation in yields of kelp, 1916, 1917, and 1918. ‘The small figures 
refer to yields of wet kelp in hundreds of tons per square mile of area harvested. 
Soft crumbling rocks can not hold them. A great movement of sand 
may cover the rocks, to which kelp plants have been attached, so deep 
that new plants can not get a foothold there; and, on the other hand, 
by a similar movement of sand, rocks may be uncovered and, with 
other conditions favorable, kelp plants may take possession of them. 
DEPTH. 
Setchell has discussed the influence of depth on the growth of 
Macrocystis to which discussion may. be added the statement that the 
plant makes its best growth in 40 or 50 feet of water, the heaviest mats 
and the longest fronds being always found overlying water of this 
depth. Plants growing in water deeper than this usually have much 
the same appearance as land plants grown in excessive shade. They 
apparently find it difficult to reach the surface, even when the water 
is still, and when there is a strong current they are often drawn far 
under and can not get the benefit of light. On the other hand, plants 
growing in shallow water lack vigor. Often.their fronds are too 
short for profitable harvesting. : 
