ANGIOSPERMS. 
577 
of Orchids, only a single cell is thus divided, in other cases several. By repeated 
divisions parallel to the first the cells are multiplied, and a protuberance is formed 
consisting in Orchids (Fig. 397) of a single row of cells, in other cases of a number 
of rows, invested by the epidermis ; this protuberance is the nucellus. 
The general course of the development of the embryo-sac is as follows : the 
terminal cell of the row in simple ovules (Orchids, Monotropd), the terminal cell of 
the axial row in more complex ovules, a hypodermal cell therefore ^ becomes distin- 
guished by its size and by the granularity of its protoplasm. This cell elongates with 
the growth of the nucellus, and a segment is cut off from it towards its upper 
(micropylar) end by a transverse wall, and this may be followed by the cutting off of 
a second segment in a similar manner ; only one such segment is cut off in Tritonia 
aurea, Anther icuvi ramosum, Triglochin palustre^ Luzula pilosa, Tradescantia vi'rgimca, 
Chenopodium foeiidum, Helianthemum Rhodax, etc., two in many Rosaceae : when 
only one segment is cut off, it usually divides into two by a wall parallel to the long 
axis of the nucellus (antichnal), and whether one or two segments have been primarily 
cut off, they may undergo division by transverse walls. The large remaining cell now 
usually is divided by a transverse wall into two of nearly equal size, and one or both 
of these may be divided in a similar manner. The walls which are formed in connexion 
with these divisions are remarkable for their thickness and their glistening appearance. 
The result of these divisions is the formation of a row of three or four cells lying 
in the long axis of the nucellus ; it is usually the lowest cell of this axial row which 
enlarges and becomes the embryo-sac, causing by its growth the absorption of 
the others. 
Before going into further detail it will be well to become acquainted with the 
terminology which is to be used in describing these phenomena, and this may be best 
done by comparing them with those which accompany the development of the 
sporangia in the Vascular Cryptogams. It has been already pointed out that the ovule 
corresponds to a sporangium, and we see, from the facts stated in the preceding 
paragraph, that in its first development it resembles the sporangia of Isoetes in that it 
is derived not from the epidermis only but also from subjacent cells, and that, as in 
the sporangia of the majority of Vascular Cryptogams, there appears within it at an 
early stage a hypodermal cell which is readily distinguishable from the cells surround- 
ing it : to this cell the term archesporium may be applied as well here as in speaking 
of the Vascular Cryptogams, Similarly we may call the cell or cells which are cut 
off from the archesporium toward its micropylar end iapetal cells, the tapetum being 
completed in these plants by cells of the nucellus, a condition which recalls that in 
Selaginella. The further divisions of the archesporium are comparable to those which 
take place in the sporangia of the Vascular Cryptogams and which result in the forma- 
tion of the mother-cells of the spores : the axial row of cells is then a row of spore- 
mother-cells, and, inasmuch as one of these developes into the embryo-sac, the 
of Orchids are developed in the manner described above in the text. Warming also states that in 
many cases the first cell-divisions make their appearance in the layer next but one to the epidermis 
{Ribes, Viola, Ficaria, Geum, Lamium, Symphytum, Verbascum), or even in a deeper layer (Malva, 
Pisum).'] 
^ [In Carex prcecox, according to Fischer, the archesporium is derived from a more deeply 
placed cell of the nucellus.] 
P p 
