. Popular science monthly. ime as was done by the ferns;and in our estimation this test represents a fairly good duplication ofthe fern phenomenon. If we consider the average cross section area ofthe six fern fronds and divide this by the total weight lifted, we findthat the cells of the young fronds exerted about 35 atmospheres to over- TEE POWER OF GROWTH IN PLANTS 235 come the resistance offered by the concrete. This we consider a veryfair estimate, although from our other experiments we are led to believethat as high as 50 atmospheres are sometimes required to accomplishthe work with the c
. Popular science monthly. ime as was done by the ferns;and in our estimation this test represents a fairly good duplication ofthe fern phenomenon. If we consider the average cross section area ofthe six fern fronds and divide this by the total weight lifted, we findthat the cells of the young fronds exerted about 35 atmospheres to over- TEE POWER OF GROWTH IN PLANTS 235 come the resistance offered by the concrete. This we consider a veryfair estimate, although from our other experiments we are led to believethat as high as 50 atmospheres are sometimes required to accomplishthe work with the conditions under which the ferns were concrete was so hard that after it had been ruptured it was impos-sible to make any the ragged edges except by the use oftools. The work was done by a slow and constantly increasing pres-sure on the under surface of the concrete, the principle being somewhatthe same as in the straightening of teeth and bones, although in suchcases the pressure is not Fig. 6. Method of determining the longitudinal power of growth in roots. Theroots are held firmly in two plaster of Paris casts, and the amount of pressure indi-cated by the spring. (After Pfeffer.) At this point we might consider what growth is and how it is ac-complished in a plant. Growth is defined as a stretching and fixationof the cell walls, accomplished by osmotic pressure characteristic of thesolutions contained in the cell vacuole. In ordinary growth there is apressure of 1 to 3 atmospheres on the cell walls—a fact which can bedetermined experimentally with some degree of accuracy. It is thispressure which gives plants their rigidity and freshness, and anythingwhich destroys it, such as lack of water, causes the plant to growing organisms—annuals and herbaceous plants, for in-stance^—contain little mechanical or supportive tissue, and it is owingto the turgidity of the cells derived from osmotic pressure that they 236 THE POPULAR
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