256 The Phenomena of Morphogenesis 



their origin in single epidermal cells. These cells are not only presumably 

 but demonstrably totipotent. 



In a considerable number of forms the reproductive structures arising 

 in regeneration are not plantlets, already differentiated and ready to 

 start growth, but dormant, almost seed-like structures. In Lilium tigrinum, 

 for example, in the axils of the upper leaves there are, instead of buds, 

 hard black bulbils which fall off and produce new plants. Detached 

 scale leaves in a number of the Liliaceae form adventive buds or bulbils 

 from their bases. On leaves of Hyacinthus, removed from the plant, simi- 

 lar structures may be formed, but Naylor (1940) has shown that in this 

 case they do not come from preformed meristematic tissue but develop 

 from epidermal and subepidermal cells. 



In many ferns vegetative cells of the prothallus may produce sporo- 

 phytes by apogamy. Aposporous gametophytes are also frequently formed, 

 especially on isolated juvenile leaves. Lawton ( 1932, 1936 ) was able to 

 induce apospory in 13 species of ferns and obtained from them tetraploid 

 sporophytes by methods similar to those used in mosses. Aposporous struc- 

 tures are often strictly gametophytic, but Beyerle (1932) found adventive 

 structures in 34 fern species to include sporophytic buds, undifferen- 

 tiated structures, prothallia, and bodies intermediate between sporophytes 

 and prothallia (reviews by Du Buy and Neurnbergk, 1938, and Steil, 

 1939, 1951). 



Regeneration in the fern sporophyte often results in the production of 

 new plants. Buds may be formed on leaves and roots, and foliar em- 

 bryos resembling those of higher plants may be produced, as in Campto- 

 sorus ( McVeigh, 1934; Yarbrough, 1936£> ) . A review of reproductive re- 

 generation in ferns, covering 35 genera and 197 species, has been made 

 by McVeigh (1937). 



There should also be mentioned a special kind of reproductive re- 

 generation that occurs in vegetative tissues of the embryo sac and ovule, 

 by which embryos develop from synergids, antipodal cells, or cells of 

 the nucellus and not only from the fertilized egg (Lebeqne, 1952). 

 Strasburger (1877) called this adventive polyembryony (as opposed to 

 cleavage polyembryony, p. 235 ) . Examples of this are known in Funkia, 

 Coelebogyne, Citrus, and others (review by Webber, 1940). The em- 

 bryos here formed somewhat resemble the foliar embryos of the 

 Crassulaceae and similar plants. Though sometimes started in their 

 development by a regenerative stimulus, these are cases of specific repro- 

 ductive processes made possible by the totipotence of cells in various 

 vegetative portions of the plant and which in a way are comparable to 

 asexual propagation by spores. 



In recent years experimental work on morphogenetic problems has 

 been concerned primarily with the effects of environmental factors, 



