Harmony and Conflict in the Living World - Alexander F. Skutch [3]
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A Realm of Paradoxes
What is the most fundamental difference between a living organismyourself, for exampleand a lifeless object, such as a stone? You are self-moved, and the rock is not. You feel and think, as stones evidently cannot do. You are structurally much more complex than any mineral, and your parts are more closely integrated. You are capable of doing a hundred things that stones cannot do. We might continue for pages to enumerate all the ways in which people, and other living things, differ from lifeless things, without hitting upon the most basic difference because it is perhaps the least obvious. I hope that you will not be offended if I suggest that the most fundamental difference between you and a stone is that you are covered by skin and the rock is not. All that the living world has achieved, all its glories and likewise its tragedies, may be traced to the unexciting and sometimes overlooked fact that organisms of all kinds are separated from their ambience by a semipermeable integument, a thin pellicle or a thick skin, such as inorganic objects commonly lack.
Insulation and its Consequences
The basic unit of life is the cell, with the protoplast that it encloses. This consists of the watery, somewhat viscous cytoplasm and the organelles within it, including a nucleus, mitochondria, and various
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other plastids. Even one-celled organisms, scarcely visible or invisible to our unaided eyesthe amoeba and the paramoeciumare vastly complex. To carry on their diverse functions, they must control their contents, retaining within themselves what they need, resisting the intrusion of superfluous or harmful substances from the surrounding water. They cannot completely insulate themselves from their milieu, for they depend upon it for indispensable materials, and they must return to it waste products of metabolism that would be injurious if permitted to accumulate. To control its exchanges with its surroundings, each minute organism encloses itself in a selectively permeable pellicle or membrane, which freely permits the inward or outward diffusion of certain substances but refuses passage to others. The creature's life depends upon the maintenance of this exceedingly thin and fragile barrier. To impair it is to kill the organism.
Plants and animals increase in complexity by adding cell to cell. Although they cooperate closely, the cells of a multicellular organism preserve a certain independence by retaining the semipermeable ectoplasm that regulates their exchanges with adjoining cells. This is most readily demonstrated in vegetable tissues with cells enclosed in more or less rigid walls of cellulose. Tender growing stems and leaves maintain their shapes while their cells are turgid with water; if they lose too much liquid they wilt and droop, like a balloon from which the air escapes. If one places a thin section of plant tissue in a concentrated solution, as of cane sugar, and watches through a microscope, each protoplast can be seen shrinking away from its enclosing wall of cellulose. The cell's semipermeable ectoplasm permits water to flow outward but retards or prohibits the inward diffusion of the solute. The cytoplast continues to lose water and contract until its osmotic pressure equals that of the solution in which it is immersed.
In addition to the defenses of their individual cells, multicellular organisms develop more obvious and resistant means of regulating their exchanges with their media. Trunks, branches, and roots of woody plants cover themselves with bark, which at least on younger branches is penetrated by lenticels more permeable to air. Leaves
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and herbaceous stems are covered with waxy cuticles, which are thicker and less permeable to water the more arid the environment. Penetrating the cuticle and epidermis of leaves and green stems are multitudes of minute pores, the stomata, which by opening and closing regulate