THE ORIGIN AND DEVELOPMENT OF THE EMBRYO 
SAC AND EMBRYO OF DENDROPHTHORA 
OPUNTIOIDES AND D. GRACILE. II 
HARLAN HARVEY YORK 
(WITH PLATE VII) 
Embryo-formation in D. opuntioides 
Following the arrangement of the nuclei in the micropylar end 
of the sac, as described above, they enlarge, the one toward the 
so-called polar nuclei becoming the largest. This nucleus becomes 
the one functional cell from which the endosperm and embryo 
are derived. Later the “synergid” and the polar nuclei begin to — 
degenerate and in a short time disappear. Preparatory to division, 
the functional nucleus becomes very much enlarged and ‘divides 
by ordinary mitosis, in a plane transverse to the longitudinal axis 
of the sac (fig. 46). This mitotic division was the first observed 
following that of the megaspore mother cell. It was not possible 
to count the chromosomes, but the number was apparently equal 
to that seen at the division of the megaspore mother cell. The 
two cells resulting from the division of the functional nucleus 
divide transversely to the plane of the first division. After a series 
of divisions a large oval mass of tissue is formed which occupies 
the upper end of the sac. The cells of this body are large and 
appear to be exactly alike (fig. 47). After this endosperm-like 
body has reached about one-twentieth of its size in the mature 
seed, the cytoplasm of one of the central cells becomes more densely 
granular than in the others. This cell constitutes the one-celled 
stage of the embryo (fig. 48). It divides by a wall almost trans: 
verse to the longitudinal axis of the flower, forming a 2-cell 
embryo (fig. 49). A division of these two cells in a plane trans 
verse to the wall which separates them results in the formation of 
the 4-celled embryo (fig. 50). 
The embryo thus arises indirectly from a single cell without 
fertilization having occurred. The author examined carefully 
more than 500 plants of D. opuntioides, occurring within a radius 
Botanical Gazette, vol. 56] [200 


