THE CARPEL AND OVULE 



261 



upgrowth : it consists internally of a number of radial rows of cells, covered 

 by a superficial layer. The latter divides in Caltha into two or more layers at 

 the tip of the nucellus, forming a cap of tissue covering the radial rows within. 

 It is from the central row of the internal cells that the embryo-sac arises. The 

 condition in Caltha is relatively simple. The terminal cell of the central row 

 undergoes division into two, and then into four (Fig. 207). This is in fact a 

 tetrad-division, and the mother-cell which divides is of hypodermal origin. 

 It has been shown that this division is accompanied by reduction of chromosomes 

 of the nuclei to the half number, as in the pollen-tetrad. The resulting four cells 

 are arranged in a row ; pollen-tetrads are sometimes found to have the same 

 arrangement. The conclusion follows that the tetrad thus produced in the ovule 

 is the correlative of a single pollen-tetrad, and each of the cells might become a spore. 

 This actually happens in the pollen-sac ; but in the ovule as a rule only one 

 of the four potential spores develops further. The lowest 

 cell of the four enlarges at the expense of the others, 

 which collapse, and are crushed out of shape. Ihe 

 embryo-sac encroaches slso on the surrounding cells of 

 the nucellus, which give way to allow of its increase in 

 size. Thus, as shown by its development, the embyyo- 

 sac is the single spore of a tetrad : as it develops to a 

 large size it is styled a mega-spore. 



In other cases the structure may be more complex than 

 in Caltha. A considerable number of Flowering Plants 

 show rapid growth and division of the superficial cells 

 at the tip of the nucellus, so as to form a considerable 

 pad of tissue covering the hypoderma. This is seen in 

 Rosa livida (Fig. 208), which also shows numerous hypo- 

 dermal cells with dense contents, each divided into a 

 cell-row. The basal cell of each row is an embryo-sac 

 mother-cell ; the distal cells are parietal cells, com- 

 parable with the parietal cells of the pollen-sac (compare Fig. 195). There 

 is thus in Rosa a plurality of mother-cells, as is usual in pollen-sacs ; while 

 the parietal cells, which are absent in Caltha, are here present, and strengthen 

 the comparison with the pollen-sac. Such conditions, which are not infrequent 

 among the more primitive Dicotyledons, indicate that the pollen-sac and the 

 ovule, though differing in form, have essential features in common. Both 

 are sporangia, though they have diverged in details of development. 



Other ovules again are simpler than Caltha. For instance, in Monotropa 

 the nucellus is represented only by a single layer of cells, surrounding one 

 central row, which consists of the embryo-sac itself and two sister cells (Fig. 

 209, i.). Here there are no parietal cells, and the tetrad itself is represented 

 only by three cells, the uppermost cell of the tetrad not having undergone the 

 second division. The two sister cells are later disorganised, and make way for 

 the embryo-sac ; and finally the single layer of the nucellus is also absorbed 

 (Fig. 209, viii.) ; so that at fertilisation the embryo-sac is all that represents the 

 nucellus. A still simpler condition as regards the origin of the embryo-sac 

 though not of the whole structure, is seen in Lilium. Here, as in most Mono- 

 cotyledons, the parietal cells are absent. Further, the hypodermal cell of the 

 central row itself becomes directly the megaspore-mother-cell, and reduction 



FIG. 208. 



Young nucellus of ovule 

 of Rosa livida. See text. 

 (After Strasburger.) 



