Page:Popular Science Monthly Volume 74.djvu/590

586 bark, or rock. Their structure is dorsi-ventral. When these little plants first start, they are erect and cylindrical; the divisions of the fertilized egg-cell from which they spring are at right angles to the source of light. Presently, however, these little cylinders tip over and, the light still coming from above, they spread out at right angles to it. Thus the erect cylindrical form and radial structure soon give place to prostrate leafy form and dorsi-ventral structure. It is now known in at least one case, and suspected in many others, that if the little plants can continue to receive light symmetrically, their form will be correspondingly symmetrical. By slowly revolving them for months after sowing, so that they were equally illuminated on all sides in succession, I have obtained plants which were as cylindrical at the end of my experiment as in the early weeks. Where the illumination was equal, the structure was perfectly radial; where it was unequal, the structure was dorsi-ventral.

This matter of bodily form, different or like in two succeeding generations, depends upon the direction from which the light comes. If the offspring have a one-sided illumination, as their parents did, their form will be flat and-prostrate like their parents; but if the offspring are symmetrically lighted, they will be symmetrically formed in spite of the difference from their parents.

So far as these experiments contribute at all to the solution of biological or sociological problems they do so by indicating that like influences produce like effects on the same substance, and that, although the substance may be the same, unlike influences will produce unlike results. They make us a little more confident that the child of vicious parents, if itself sound, can be made into a much more desirable citizen if brought under influences different and better than those surrounding and exerted by its parents.

The formative influences so far discussed produce normal and healthy effects. The deformative and pathogenic influences which affect human and other animal bodies have their parallels among plants. Besides the plainly marked plant-diseases due to such obvious parasites as borers, rusts, rots and mildews, there are influences no less real, although easily overlooked.

City life is unfavorable to plants. Atmospheric and soil conditions are either bad or not bad, they are never good. One need only pass along a street in which the gas-pipes have been exposed to know that the soil is more or less saturated with stale illuminating gas. The odor is offensive. Trees rooted in soil poisoned by large or small, but always continuous doses, of illuminating gas, do not thrive. Their leaves are never the full rich green of trees in the country, the foliage yellows early and many leaves fall. Add leaky electric wires, leaky sewers, and the putrefactions going on in fouled soil, and one realizes the cause of the chronic lack of vigor of street shade trees.