Page:Popular Science Monthly Volume 63.djvu/245

Rh a Spirogyra filament of the same size and general structure growing in the same pool. Cladophora grows attached, Spirogyra is free. Compare 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 tremendous 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 (Postelsia), 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, narrow 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 comparative 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 organism 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