ANGIOSPERMS, 



579 



layers of cells are thus formed constituting a sort of cap at the apex of the nucellus : 

 this is indicated in Hippuris, and it is well seen in Delphinium, Helianthemum, and in 

 the Rosaceae ; to the last case reference has been made in the preceding paragraph. 

 In Geum urbanum, Iris Pseudacorus, and Agrostemma the whole free surface of the 

 nucellus is covered by a number of layers of cells which have been formed in this 

 manner (Warming). 



The integuments are developed as outgrowths of the nucellus. In those cases 

 in which the ovule has only a single integument, the greater part of it is developed 

 from the epidermis of the nucellus. When the ovule has two integuments, the 

 inner oiiie is developed principally from the epidermis, the outer principally from 

 the subjacent cells ^ Cell-division and growth take place iff a zone encircling the 

 nucellus, and thus the integument grows up, at first as a ring, but later as a con- 

 tinuous membrane investing it. Accordingly as this zone is narrow or broad, the 

 integument will consist (in thickness) of one or more layers of cells. As a general 

 rule the inner integument in those cases in which two are present is developed 

 first ; exceptions to it appear to occur, according to Warming, in Euphorbia^ Cuphea, 

 Mahernia glabrata. 



In some cases three integuments appear to be present {Asphodelus luteus and 

 creticuSy Reseda luted), but it seems probable that the external one is to be regarded 

 as an arillus. On the other hand it appears, in some cases in which two integuments 

 are developed, as if only one were present {Viola, Ficus, Convallaria, Orchis, Tro- 

 pcEolum, Delphinium)', this is due to the very intimate connexion of the two in- 

 teguments.] 



The further behaviour of the embryo-sac of Angiosperms differs in many ways 

 from that of Gymnosperms. In Gymnosperms it remains surrounded by a thick 

 layer of the tissue of the nucellus till after fertilisation has taken place ; it is com- 

 paratively small, and is surmounted by a strongly developed nuclear protuberance. 

 In Angiosperms, on the other hand, the embryo-sac has grown considerably even 

 before fertilisation ; it usually supplants the surrounding tissue of the nucellus so far 

 that it remains enveloped by only a thin layer of it, or is even in actual contact with 

 the inner surface of the inner integument, as in Orchideae (Fig. 397, VII). In such 

 cases the tissue of the apex of the ovule often still remains entire (as in Aroideae), 

 but frequently the apex of the embryo-sac bursts through it, and projects into the 

 micropyle (as in Crocus and Labiatae), or even grows out beyond it as a long tube 

 {e.g. Santaluni). The middle and lower part of the sac also frequently extends con- 

 siderably; in many gamopetalous Dicotyledons it puts out vermiform appendages 

 which penetrate into and destroy the tissue of the integument, as in Rhinanthus, 

 Lathrcea, and some Labiatae. While this process of growth is proceeding, the proto- 

 plasm which at first fills up the whole sac becomes full of vacuoles ; a large sap- 

 cavity arises surrounded by a parietal mass of protoplasm, which accumulates 

 especially in the apical prominence and at the bottom of the embryo-sac, while 



^ [According to Warming the integuments are developed in some cases from the dermatogen 

 alone ; as in Orchis, Primula chinensis, Centradenia floribunda, Lysimachia verticillata, Linnea perenne 

 (Strasburger), and Begonia heracleifolia, among plants which have two integuments, and in Peperomia 

 and Monotropa among those which have only one integument. This mode of development of the 

 integuments is comparatively rare.] 



P p 2 



