214 The Phenomena of Morphogenesis 



of the ontogenetic cycle is usually reversible in the sense that cuttings 

 taken from any part of the shoot system or at any stage of development 

 will, by regeneration, produce a normal plant. In some cases, however, 

 changes at the growing point have been so great that the newly developed 

 structures seem to have undergone irreversible modification. A notable 

 example of this is the English ivy described in the preceding paragraph, 

 for in this plant cuttings made from the flowering shoot rarely revert to 

 the climbing form but instead produce upright, radially symmetrical 

 plants, the variety arborea of horticulture. These are often used as dwarf, 

 tree-like ornamentals but usually die after a few years. No genetic change 

 is involved here, for seeds produce the climbing, lobed form again. There 

 has been a good deal of debate as to the complete irreversibility of this 

 change (Bruhn, 1910; Furlani, 1914), but the usual behavior is the one 

 just described. Kranz (1931), however, finds that the transition from 

 juvenile to adult foliage is often not a sudden one but that the five-lobed 

 type gives place to a three-lobed one before the mature, ovate leaves are 

 formed. Robbins (1957) was able to change the adult form of foliage to 

 the juvenile one by treatment with gibberellic acid. 



A somewhat similar case is the persistence of juvenile structures which 

 can sometimes be induced by growing cuttings from the seedling stem. 

 The most notable example of this is found in certain of the cypress-like 

 conifers, where the seedling leaves are needle-like but are soon followed 

 by the scale-like foliage characteristic of the species. If cuttings are made 

 from the lateral branches arising just above the cotyledons in Thuja, for 

 example, they will produce plants, often growing to small trees, which 

 bear nothing but the needle-like juvenile foliage, the horticultural variety 

 "Retinospora" (Beissner, 1930). Such plants do not flower and are rela- 

 tively short-lived. In some way, severance of the juvenile shoot from its 

 roots seems to have prevented completion of the normal ontogenetic 

 cycle. Other cases have been reported in which seedlings used as cuttings 

 grow very differently from those which are left on their own roots 

 (M. R. Jacobs, 1939). Beissner's results were challenged by Woycicki 

 (1954), who grew cuttings from seedlings of Thuja, Chamaecyparis, and 

 Biota but found that the juvenile foliage did not persist. He believes that 

 the Retinospora forms arose by spontaneous mutations in seedlings or 

 young shoots. 



Whatever the facts in this case may be, others have been reported in 

 which it is certain that cuttings taken from various parts of the plant 

 produce individuals different from the normal type and like the part of 

 the plant from which they came and in which these differences persist 

 during the life of the cutting, or at least for a long time, but do not involve 

 genetic change. This phenomenon Molisch ( 1930 ) termed topophysis. A 

 familiar example occurs in Araucaria (p. 189), where the flattened, dor- 



