338 FOURTH GROUP.— SEED-PLANTS. 



embryo ; there is no separation of the tubes of the suspensor. In Gingko, where the 

 formation of the embryo does not begin till after the fall of the ovules from the tree, the 

 germ-nucleus in the oospore first divides, and the repeated division of the daughter-nuclei 

 gives rise to a large number of separate cell-nuclei, which lie free in the protoplasm of 

 the oospore. When the fixed number of these cells is completed, they become 

 surrounded with threads of protoplasm and cell-walls are formed between them, and 

 the entire oospore appears to be filled with a body of tissue which forms the embryo. 

 In this case therefore only one embryo is formed in each archegonium, and a true 

 suspensor is not formed ; it is only indicated by the cells which are towards the neck 

 of the archegonium developing into short cells. In Cephalotaxus Fortunei and 

 Araiicaria brasih'ensis, as Strasburger has recently ascertained, the growing point of 

 the embryo is not the apex of the rudiment but is formed inside the rudiment, while 

 the original apex serves only as an organ of penetration and protection, and is 

 afterwards thrown off. 



It appears then that in the Coniferae one or more embryos are formed from one 

 oospore, and their number inside a prothallium (endosperm) is increased by the 

 circumstance that several archegonia are fertilised at the same time ; polyembrony 

 therefore, which is exceptional in Angiosperms, is the rule among the Coniferae and 

 generally among Gymnosperms, but only in the rudimentary state, for usually one 

 rudiment only developes into a vigorous embryo, such as has been described above. 

 The endosperm also grows rapidly during the development of the embryo, and its 

 cells become filled with reserves of food-material (fat and albuminous substances) ; the 

 embryo-sac also which incloses it enlarges at the same time and ultimately displaces 

 the tissue of the nucellus, while the tissue of the integument hardens into the seed- 

 coat ; in Gingko a thick outer layer of tissue produces the pulpy investment which 

 makes the seed resemble a plum (drupe). The tubular cells of the suspensor usually 

 disappear during these processes, but are said by Schacht to persist in Larix. 



The Coniferae like the Cycadeae are essentially distinguished from the Vascular 

 Cryptogams by the presence of a primary root. In the young pro-embryo the 

 lower part which is rich in protoplasm, the embryo proper, is clearly distinguished 

 from the upper part, the suspensor. The differentiation of the root begins in 

 Thuja, when the lower part of the rudimentary embryo has reached a length of four- 

 tenths of a millimetre ; it commences at some depth in the tissue of the embryo, 

 about a tenth and a half of a millimetre below its apex. Tangential divisions first 

 appear in a layer of cells disposed in a half circle and enclosed on all sides by the 

 tissue of the rudimentary embryo; consequently the first mdiment of the root is 

 covered from the beginning on the side towards the suspensor by several layers of 

 cells. The differentiation of the root proceeds in a similar manner in the rest of the 

 Coniferae. The cotyledons are formed beneath the apex of the rudimentary embryo 

 as was stated above. 



While the seeds are ripening the parts in their vicinity are subjected to further 

 growth and changes of consistence ; in Taxus the aril, which afterwards becomes 

 red and pulpy, grows up round the seed as it ripens (Fig. 252 F ni); in Podocarpus 

 the aril, which had been formed before, becomes pulpy ; in Jimiperus and Sabitia 

 the united bract-scales, which had contracted, develope into the blue berry which 

 encloses the seeds; in most of the other Cupressineae and in the Araucarieae the 



