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 Lilhmi bulbi/erum, 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 {Myosurus minimus^ Senecio vulgaris^ Lamium maculatam. Delphinium, tridac- 

 tylon and villosum, Sisyrinchium iridifoliuvi, Orchis pallens, Gymnadenia conopsea, 

 Monotropa Hypopitys, and several Grasses) no tapetal cells are formed. In Alisma 

 Plan/ago, Allium Jislulosuni^ Chenopodium foetidum, 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 japonica, 

 Geum slrictum, Sanguisorba pratejisis^ 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 Cruciferac 

 {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. Sci. Nat. XII, 1849.] 



