406 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



as the tropical screwpine Pandanus puts out cables and shrouds to 

 support the increasing weight of the growing head of branches. The 

 branching false roots of the Laminaria are merely compound Fucus 

 disks and in some few instances, as in Macrocystis^ the grasping fibers 

 of the rootlike part develop extensively and form a matted sub- 

 stratum from which many stemlike parts originate. The holdfast 

 extends over the flat surface and adheres to it with no tendency to 

 penetrate it as do the roots of higher plants. Only on unstable soil, 

 as on the shores of the Florida Keys, do the rootlike holdfasts of the 

 Siphoneae and the Caulerpeae penetrate into the sand, forming a 

 compact cushion but in search of stability in the shifting sands rather 

 than for food. 



The four chief varieties of color observed among the algae are 

 green, blue-green, olive green or brown, and red. They form an easy 

 method for separating the algae into separate divisions since the 

 classes of color are fairly constant among species that are allied phys- 

 iologically and morphologically. 



The green color of the Chlorophyceae, or green algae, and the 

 blue-green color of the Cyanophyceae, or blue-green algae, is char- 

 acteristic of the algae that grow either in trees, in the soil or on 

 land, on rocks, in fresh water, and in the shallower parts of the sea 

 where they are exposed to full sunshine but are seldom quite un- 

 covered by water. About one-fourth of the green and blue-green 

 algae found in deep water are as vivid a green as those found near 

 the surface so that it cannot be assumed that the green color, as in 

 land plants, is due to a perfect exposure to sunlight. 



The brown algae, or Phaeophyceae, are most abundant between 

 tide marks in places where they are exposed to the air at the recess 

 of the tide, and are thus alternately parched in the sun and flooded 

 by the cool waves of the returning tide. They may extend to low- 

 water mark and form a broad belt of vegetation about that level. 

 A few straggle into deeper water, sometimes into really deep water. 

 The gigantic deep-water algae Macrocystis^ Nereocystis, Lessonia, 

 and Durvillea are olive-colored. 



The red algae, or Rhodophyceae, are most abundant in the deeper 

 and darker parts of the sea. They rarely grow in tide pools with 

 the exception of pools shaded from the direct rays of the sun by an 

 overhanging rock, or by overlying brown algae. The red color is 

 always most intense and pure when the plant grows in deep water 

 as may be observed by tracing the same species from the greatest 

 depth to the least depth at which it is found. Ohondrus crispus is 

 deep-purplish red in deep pools near the low-water mark but in 

 shallow pools where it is exposed to the sun's rays it fades to greenish 

 or whitish shades. 



