GYMNOSPERMA E. 311 



The embryo-sac is marked by its thick wall, which sometimes, as in the Cycadeae, is 

 furnished with a cuticularised outer layer corresponding to the exosporium. The 

 prothallium or endosperm is formed in the embryo-sac some time before fertilisation 

 by the formation of free cells, which soon however unite to form a connected tissue 

 and multiply by division; the prothallium therefore is endogenous as in the 

 Selaginelleae, and archegonia are formed on it in larger or smaller numbers. 

 According to Strasburger each archegonium originates in a cell of the pr.othallium 

 lying at the apex of the embryo-sac, which increases considerably in size and 

 produces by division the neck and central cell of the archegonium ; a small upper 

 portion of the large central cell is separated off to be the canal-cell, as in the Vascular 

 Cryptogams, while the other and larger part becomes the oosphere. When the 

 pollen-tube has grown through the tissue of the nucellus and reached the archegonium, 

 and the oosphere has received the fertilising substance from it, the embryo is formed 

 in the oospore, the whole of which is employed for the purpose, as in Gingko, or 

 only the lower portion of it 1 . The cells of the suspensor are at first small, but the 

 middle or upper ones grow out into long tubes, which pushing the lower ones before 

 them break through the lower part of the archegonium and make their way into a 

 softened part of the prothallium. Sometimes adjoining tubes of the suspensor 

 separate from one another, and each produces at its apex a small-celled rudiment of 

 an embryo ; from this cause and because the oospheres in several archegonia are 

 often fertilised in one prothallium, the immature seed may contain several rudimen- 

 tary embryos (polyembryony), but one embryo only as a rule is developed and the 

 others dwindle away. 



During the development of the embryo the prothallium becomes filled with 

 food-material and increases considerably in size, and the embryo-sac which encloses 

 it grows with it and at length entirely supplants the surrounding tissue of the nucellus ; 

 the integument or an inner layer of it is developed into a hard seed-coat, and in 

 naked seeds the outer tissue not unfrequently becomes fleshy and pulpy and gives 

 the seed the appearance of a plum-like fruit, as in Cycas and Gingko ; sometimes 

 the effects of fertilisation extend to the carpels and other parts of the flower, which 

 grow vigorously and form fleshy or woody envelopes round the seeds, or cushions 

 underneath them. 



The ripe seed is always filled with endosperm (prothallium) in which the embryo 

 lies, distinctly differentiated into stem, leaves and root, and filling an axile cavity in 

 the endosperm ; it is always straight, with the tip of the root towards the micropylar 

 end and the points of the leaves towards the base of the seed. The first leaves 

 produced by the stem of the embryo are in a whorl consisting generally of two 

 opposite members, but not unfrequently of three, four, six, nine, or even more. When 

 the embryo unfolds in germination, the tip of the root first protrudes through the 

 bursting seed-coat; by the elongation of the cotyledons or first leaves the bud 

 formed between them at the apex of the stem is thrust forth, but the cotyledons 



1 The Gymnosperms afford the only examples in the vegetable kingdom of a phenomenon 

 which is widely spread in the animal kingdom, namely, the formation of the embryo from a part 

 only of the oosphere (meroblastic formation), though we cannot distinguish in them between the 

 ' formative and nutritive yolk.' 



