54
JOHN G. TORREY
ture embryos from the ovule, and, in fact, makes possible the handling of
large numbers of single cells potentially able to develop into normal
embryos. Thus, tissue culture and embryo culture techniques come together. With the availability of numerous adventive embryos at different
developmental stages induced in callus tissue cultures, it has now become possible to do quantitative analyses of the general type illustrated
in the work discussed above (Section ΙΙ,Β). With such embryos it should
be possible to combine nutritional studies with physiological and histochemical analyses. Thus the way is open in these species at least to
explore much more completely the biochemical events associated with the
early stages of initiation of embryo organization.
It remains to be seen whether coconut milk contains unique hormonelike factors which are essential to the embryo induction in callus tissue.
Halperin's work with defined media (1964) would suggest that it does
not. Coconut milk contains auxins, probably IAA, and perhaps others;
it contains one or more substances that act as kinins, but which have
yet to be identified; it also contains purines and pyrimidines. All these
constituents have been shown to be active at very low concentrations in
morphogenetic phenomena. In addition, the gibberellins may play some
role in embryo development. More (and difficult) chemical analysis is
still needed, as has been stressed by Steward and Shantz (1959). Enough
is known of synthetic nutrient media from tissue culture and from embryo culture studies, however, to predict that defined media for the
manipulation of isolated somatic cells and their induction into embryos
will be forthcoming in the not too distant future.
What causes a cell to develop into an embryo? One cannot attribute
somewhat mystical directive forces to the special environment of the
embryo sac, since it can be dispensed with under some experimental
circumstances. Steward has emphasized that the essential feature in the
process is the isolation of the cell from intimate association with its
neighboring cells, freeing it from the correlative influences of adjacent
tissues in contact with it. Such a cell in isolation can express its intrinsic
potentialities. The fact that such a cell can go on to form a whole
plant and pass through recognizable genetically controlled stages on the
way to its adult form demonstrates without doubt that such a cell, even
if derived from differentiated somatic cells of an adult, has the full
genome of the original zygotic nucleus and has survived through differentiated states without genetic loss. There has been no irreversible
nuclear differentiation in the sense of Briggs and King (1957). We have
no data on the proportion of carrot cells in a callus population which can
go on to form a whole plant. In the earlier studies of Steward et al.
(1958), only a small proportion of cells actually organized to form roots
JOHN G. TORREY
ture embryos from the ovule, and, in fact, makes possible the handling of
large numbers of single cells potentially able to develop into normal
embryos. Thus, tissue culture and embryo culture techniques come together. With the availability of numerous adventive embryos at different
developmental stages induced in callus tissue cultures, it has now become possible to do quantitative analyses of the general type illustrated
in the work discussed above (Section ΙΙ,Β). With such embryos it should
be possible to combine nutritional studies with physiological and histochemical analyses. Thus the way is open in these species at least to
explore much more completely the biochemical events associated with the
early stages of initiation of embryo organization.
It remains to be seen whether coconut milk contains unique hormonelike factors which are essential to the embryo induction in callus tissue.
Halperin's work with defined media (1964) would suggest that it does
not. Coconut milk contains auxins, probably IAA, and perhaps others;
it contains one or more substances that act as kinins, but which have
yet to be identified; it also contains purines and pyrimidines. All these
constituents have been shown to be active at very low concentrations in
morphogenetic phenomena. In addition, the gibberellins may play some
role in embryo development. More (and difficult) chemical analysis is
still needed, as has been stressed by Steward and Shantz (1959). Enough
is known of synthetic nutrient media from tissue culture and from embryo culture studies, however, to predict that defined media for the
manipulation of isolated somatic cells and their induction into embryos
will be forthcoming in the not too distant future.
What causes a cell to develop into an embryo? One cannot attribute
somewhat mystical directive forces to the special environment of the
embryo sac, since it can be dispensed with under some experimental
circumstances. Steward has emphasized that the essential feature in the
process is the isolation of the cell from intimate association with its
neighboring cells, freeing it from the correlative influences of adjacent
tissues in contact with it. Such a cell in isolation can express its intrinsic
potentialities. The fact that such a cell can go on to form a whole
plant and pass through recognizable genetically controlled stages on the
way to its adult form demonstrates without doubt that such a cell, even
if derived from differentiated somatic cells of an adult, has the full
genome of the original zygotic nucleus and has survived through differentiated states without genetic loss. There has been no irreversible
nuclear differentiation in the sense of Briggs and King (1957). We have
no data on the proportion of carrot cells in a callus population which can
go on to form a whole plant. In the earlier studies of Steward et al.
(1958), only a small proportion of cells actually organized to form roots