LAND AND WATER PLANTS. 241 



a Spirogyra filament of the same size and general structure growing in 

 the same pool. Cladophora grows attached, Spirogyra is free. Com- 

 pare Nereocystis and Macrocystis, the great kelps of the Pacific, with 

 the Sargassum of the Atlantic. Sargassum begins life as an attached 

 plant but is mechanically weak, is broken away and is for most of its 

 life free. Our Pacific kelps are always attached and are tremendously 

 tough. The comparison is not fair, however, for Sargassum is smaller 

 than our giant kelps. 



The attached plants between the tide-marks are among the most 

 interesting as to mechanical strength. The rock weeds (Fucus), the 

 Irideas, the Gigartinas, etc., of our Pacific shore withstand a tre- 

 mendous amount of pulling and buffeting and are very hard to pull, 

 though comparatively easy to tear, to pieces. These and other thinner 

 and more delicate plants, e. g., the Ulvas, Porphyras, etc., escape 

 destruction by their extreme pliancy rather than by toughness. 



The most striking example of mechanical strength displayed by 

 any plant living between the tide-marks is furnished by the sea palm 

 (Postelsiu) , which is peculiar to the Pacific coast. This plant grows to 

 a height of twelve to eighteen inches. The erect and smooth tapering 

 trunk rises from the tangled mass of holdfasts attaching it to the flat or 

 shelving ledge. The leaves, often over half as long as the trunk, nar- 

 row and corrugated, spring from its top. The trunk is like that of an 

 erect land plant in being able to support a considerable weight applied 

 vertically. The sea palm resembles in carrying power the land plant 

 which gave it its name, but its remarkable strength is shown by its 

 living where almost nothing else can, where the constantly beating surf 

 is too much even for barnacles unless they take hold in some crevice. 

 The spores must germinate very rapidly in the short times of compara- 

 tive quiet, taking fast hold of the rock, for in most places where T have 

 seen the sea palm growing, the waves were constantly in motion, and 

 usually so violent, even at low water, that a man would be carried off 

 his feet almost instantly. The sea palm bows before a breaker, bends 

 away from it, resists its downward crushing force, holds on and holds 

 together in spite of the shoreward thrust and seaward pull, thrives 

 only where the sea is roughest, is the only plant where it grows every 

 part of which has not fast hold of the rock. 



Turning from the relative buoyancy of air and water and the 

 effect of this difference in the supporting tissues of land and water 

 plants, we may examine the relative ease with which land and water 

 plants obtain their food-materials. The means by which any organ- 

 ism takes food or food materials into its living cells are simple though 

 not generally enough understood. Only when the aqueous solution m 

 the cell, permeating all its parts including the wall, is in contact with 



VOL. LXIII. — 16. 



