206 The Phenomena of Morphogenesis 



are involved here, either as causes or concomitants. There seems to be a 

 major physiological change involved in this shift from vegetation to repro- 

 duction. An important problem in both physiology and morphogenesis is 

 to find what is involved in this shift. To solve it would throw light on one 

 of the major formative processes in the plant. 



Embryology and Juvenile Stages. The science of embryology in the 

 higher plants, in the sense in which it has been developed in animals, can 

 hardly be said to exist. The early embryo is relatively inaccessible and 

 is simple in structure. The divisions immediately following fertilization 

 have been studied for many plants by Soueges (1939) and Johansen 

 ( 1950; Fig. 8-15) and show differences in certain groups, but little as pre- 

 cise as the early stages in animal embryology is to be seen. Toward the 

 micropylar end of the ovule the young radicle begins to differentiate and 

 forms an apical meristem at its tip. At the other pole, in gymnosperms and 

 dicotyledons, arise the cotyledons, with the first bud between them. The 

 monocotyledons have a somewhat more complex structure here but it 

 follows the same general course. 



Especially important to students of morphogenesis is Wardlaw's book 

 on Embryogenesis (1955a), which discusses embryogeny throughout the 

 plant kingdom, with particular emphasis on the factors that determine 

 development. Maheshwari (1950) has written a general survey of angio- 

 sperm embryology, including a useful discussion of experimental embryo 

 culture. The ability to take embryos out of the ovule at a very early stage 

 and grow them in culture has opened up a wide field of investigation 

 which should be fruitful for morphogenesis. 



Several facts of significance, particularly for regeneration, have come 

 from a study of plant embryos. In a number of cases the young embryo 

 may spontaneously divide into several parts each of which apparently has 

 the capacity to develop into a whole plant. Such cleavage polyembryony 

 has been studied by Buchholz and others (p. 235). In certain plants, 

 notably some members of the citrus family, embryos may arise not only 

 through a sexual process but by budding from the tissues of the nucellus. 

 Such nucellar embryos are important for genetics as well as for morpho- 

 genesis. 



Of particular interest, however, are those forms in which the early struc- 

 tures are markedlv different from later ones and in which characteristic 

 "juvenile" stages can be seen. This type of development has been termed 

 heteroblastic by Goebel in contrast to the more gradual homoblastic type. 

 The difference is particularly conspicuous in the character of the leaves, 

 which in the seedling are often quite unlike those of the mature plant. 

 The first pair of leaves in the Eucalyptus seedling, for example, are hori- 

 zontally oriented and dorsiventral in structure though all later foliage is 

 characteristically pendulous and bifacial. The juvenile leaves of Acacia 



