578 
PHANEROGAMS, 
embryo-sac in the ovule of an Angiosperm is equivalent to one of the spore-mother- 
cells in the sporangium of a Vascular Cryptogam. 
Among the more important deviations from the above described mode of the 
development of the embryo-sac, the following may be mentioned. In Tulipa Ges- 
neriana and in Lilium bulbiferum, according to Treub and Mellink, the archesporium 
undergoes no division, but simply enlarges and becomes the embryo-sac. In a 
number of cases described by Strasburger (Angiospermen und Gymnospermen) and 
Fischer {Myostirus minimus, Senecio vulgaris, Lamium maculatam. Delphinium tridac- 
iylon and villosum, Sisyrinchium iridifolium, Orchis pallens, Gymnadenia conopsea, 
Monotropa Hypopitys, and several Grasses) no tapetal cells are formed. In Alisma 
Plantago, Allium fistulosum, Chenopodium fcetidum, and Sabulina longi/olia, the arche- 
sporium only divides once, that is, it produces a row of only two cells, the lower of 
which becomes the embryo-sac. 
It has been not unfrequently observed that the terminal cell of more than one 
of the rows of cells of which the nucellus primarily consists assumes the characters 
of an archesporial cell : in such cases the archesporium is multicellular, a condition 
which is the normal one in Isoetes among Vascular Cryptogams. This appears to 
be commonly the case among the Rosacese. In Fragaria vesca Strasburger found 
(and his observations have been confirmed by those of Fischer on Cydonia japonic a, 
Geum s trie turn, Sanguisorba prafefisis, Rubus ccBsius, and Agrimonia Eupatorid) 
several archesporial cells forming a hypodermal layer or row. Each of these 
cells behaves in the manner already described : one or two tapetal cells are cut 
off, and then the cell undergoes division so as to form a row consisting of three 
or four cells. The lowest cell of each of these rows now begins to develope into 
an embryo- sac, but the cell of the axial row developes more rapidly than the 
others, causing their absorption ; it is this cell which constitutes the embryo-sac. 
In Rosa livida Strasburger has observed that the uppermost cells of the rows, 
which usually consist of four cells, but sometimes of five or even six, develope and 
enlarge, and the second cell of the row often does the same. In the process of 
development some of these commencing embryo-sacs become absorbed, as do also 
the tapetal cells and the layers of cells which have been formed at the apex of the 
nucellus by the repeated division of the epidermal layer; one of them generally 
extends to the integument and becomes the largest embryo-sac, but the mature 
ovule contains several embryo-sacs. Tulasne has pointed out ^ that in the Crucifera) 
{Cheiranthus Cheiri) several embryo-sacs are developed, but only one of them is 
persistent. These phenomena recall the fact that in Taxus, Ginkgo, Thuja and 
Gnetum, among Gymnosperms, several embryo-sacs are at first formed.] The 
multiplicity of embryo-sacs in the ovary of Viscum cannot be included under this 
head, for the absence of the differentiation of the ovule makes it uncertain whether 
the mass of tissue in the ovary, to which we have already alluded, is to be regarded 
as the equivalent of one or of several ovules. 
[In a great number of cases the epidermis of the nucellus remains a single layer 
of cells, but not unfrequently two layers are formed at the apex of the nucellus by 
the periclinal division of the primary epidermis (dermatogen). In some cases several 
^ [Etudes d'embryogenie vegetale, Ann. d. Sei. Nat. XII, 1849.] 
