PLANTS, Regeneration in. The subject of regeneration has received much more atten tion from zoologists than from botanists. It has long been known that if the leg or tail of a salamander be cut off, a new leg or tail will be formed, or °regenerated.° If an earthworm be cut into three pieces, the anterior piece will develop a tail; the posterior, a head; and the middle will develop both a head and a tail. These illustrations indicate the general applica tion of the term, regeneration, in animals.
In plants, the term has been applied rather loosely, but in some cases a missing part may be replaced, or regenerated and the term, regen eration, might well be limited to the replacement of an organ or structure which has been re moved, as in the case of the replacement of a leg in the salamander. In some plants, if the apex of a root be cut off, the missing part will be restored (Fig. 1, A and B), but if the cut repeats an algal ancestry. In the Algae, illus trations are numerous. The common kelp, Latninaria, in its early development, has a small, flat blade, and in the adult plant the blade has simply grown larger. In Nereocystis, one of the Giant Kelps of the Pacific Ocean, the young plant has the simple blade, but the adult form is much more complex. Compare Fig. 2 A and B with Fig. 3 A and B.
The Fucaceze, the family to which Fucus, the most familiar of all brown seaweeds belongs, offer a fine illustration in the microscopic de be too far back from the apex a new root tip is not restored, but new roots develop a short distance behind the cut surface (Fig. 1 C and D). If the shoot of a common dandelion he cut off some distance below the leaves new shoots develop from the cut surface of the root. Many plants with strong, fleshy roots behave in this way. If a vigorous willow twig be cut off and the cut end be placed in soil or in water, roots will develop from the lower part, just above the cut surface (Fig. 2). If the part in the water has buds, roots may develop at the lower part of the bud.
From stumps of trees it is very common to find new shoots developing and they may even develop into trees. It used to be assumed that such shoots come from dormant buds, but it is now known that in most cases there are no such buds in the region from which the shoots origi nated. They come from the cambium or from embryonic tissue induced by the wound. In "pollarded" willows many of the shoots arise in this way, although some come from buds already present. In the cycads, buds develop from almost any wounded portion of the trunk, the bud arising from the cambium or from em bryonic tissue due to the wound. In this case, dormant or latent buds are entirely out of the question, because the cycad stem has only one bud and that one is terminal. The underground stem of the cycad, Zamia, may be cut into several pieces and shoots will develop from the cut surfaces. This case is different from that of the potato, for the potato, as it is cut for planting, has several eyes or buds in each piece from which the new plants are formed.
Many plants are propagated from cuttings. In the Begonia or Bryophyllum, the leaf is cut into several pieces and a whole new plant is developed. This behavior differs from that noted in the salamander where a new leg grows and replaces one which has been cut off, for the piece of a leaf does not replace the missing parts and become a whole leaf, but produces a bud and a root which develop into a complete plant.
While a great deal of observation and a con siderable amount of investigation has been done along the lines indicated, the term regenera tion is so elastic that botanists still use it in describing the various cases to which we have referred.
Child, C. M., (Senescence and Rejuvenescence,' (Chicago 1915) ; McCal lum, W. B., (Regeneration in Plants' (Botani cal Gazette, Vol. XL, 1905, pp. 97-120) ; Mor gan, T. H., (New York 1901; this work deals with animals, but has a chapter on regeneration in plants).