STRUCTURE OF PHANEROGAMIA 685 



matured * seed ' contains, not merely an embryo already advanced 

 a considerable stage, but a store of nutriment to serve for its further 

 development during germination. As there is nothing parallel to 

 this among Cryptogams, it may be said that reproduction by seeds, 

 not the possession of flowers, is the distinctive character of Phanero- 

 gams. The ovules, which when fertilised and matured become seeds, 

 are developed from specially modified leaves, which remain open in 

 Gymnosperms, but which in all other Phanerogams fold together so 

 as to enclose the o wiles within an ovary. Each ovule consists of a 

 nucellus surrounded by integuments which remain unclosed at the 

 apex, leaving open a short canal termed the micropyle or ' foramen.' 

 One cell of the nucellus undergoes great enlargement, and becomes 

 the embryo-sac, whose cavity is filled, in the first instance, with a 

 mucilaginous fluid containing protoplasm. At the end of the 

 embryo-sac which lies nearest the micropyle a germ -cell or odsphere 

 is developed ; in Angiosperms by free-cell-formation, but in 

 Gymnosperms indirectly after the formation of a ' corpuscle,' which 

 represents the archegone of Selaginella. By a further process of 

 free-cell-formation the remainder of the embryo-sac comes to be 

 filled with cells constituting what is termed the endosperm; and 

 this serves, like the prothallium of ferns, to imbibe and prepare 

 nutriment which is afterwards appropriated by the embryo. In 

 many seeds (as those of the Leguminosce) the whole nutritive material 

 of the endosperm has been absorbed into the cotyledons (or seed- 

 leaves) of the embryo by the time that the seed is fully matured and 

 independent of the parent ; but in other cases it remains as a ' sepa- 

 rate endosperm.' In either case it is taken into the substance of the 

 embryo during its germination. 



Elementary Tissues. No marked change shows itself in general 

 organisation as we pass from the cryptogamic to the phanerogamic 

 series of plants., A large proportion of the fabric of even the 

 most elaborately formed tree (including the parts most actively con- 

 cerned in living action) is made up of components of the very same 

 kind as those which constitute the entire organisms of the simplest 

 cryptogams. For, although the stems, branches, and roots of trees 

 and shrubs are principally composed of woody tissue, such as we do 

 not meet with in any but the highest Cryptogams, yet the special 

 office of this is to afford mechanical support ; when it is once formed, 

 it takes no further share in the vital economy than to serve for the 

 conveyance of fluid from the roots upwards through the stem and 

 branches to the leaves ; and even in these organs (in Exogens or 

 Dicotyledons), not only the pith and the cortex, with the ' medullary 

 rays,' which serve to connect them, but the * cambium layer ' inter- 

 vening between the bark and the wood in which the periodical 

 formation of the new layers both of bark and wood takes place, are 

 composed of cellular substance. This tissue is found, in fact, 

 wherever growth is taking place ; as, for example, in the growing 

 points of the root-fibres, in the leaf-buds and leaves, and in the 

 flower-buds and sexual parts of the flower ; it is only when these 

 organs attain an advanced stage of development that woody structure 

 is found in them ; its function (as in the stem) being merely to give 



