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, Monotropa), 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 icum ramosum, Triglochin palustre, Luzula pilosa, Tradescaniia vi'rgim'ca, 

 Chenopodium fceiidum, 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 (anticlinal), 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 tapetal 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, Symphyttim, 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.] 



pp 



