MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS. 353 



mucilaginous fluid containing protoplasm. At the end of of the embryo- 

 sac nearest the micropyle, a germ-cell or 'oosphere' is developed; in 

 Phanerogams generally by free cell-formation ( 226), but in Gymno- 

 sperms indirectly as the product of the formation of a 'corpusculum,' 

 which represents the archegoniurn of Selaginella ( 347). 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 nutri- 

 ment which is afterwards appropriated by the embryo. In many seeds 

 (as those of the Leguminosce) the whole nutritive material of the endo- 

 sperm has been absorbed into the ' cotyledons' (or seed-lobes) 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 ' separate albumen.' In 

 either case it is taken into the substance of the Embryo during its germi- 

 nation. 



350. Elementary Tissues. No marked change shows itself in general 

 organization, as we pass from the Cryptogauiic to the Phanerogamic 

 Series of Plants. For a large proportion of the fabric of even the most 

 elaborately formed Tree (including the parts most actively concerned in 

 living action) is made up of components of the very same kind with those 

 which constitute the entire organisms of the simplest Cryptogams. For 

 although the Stems, Branches, and Eoots of trees and shrubs are princi- 

 pally composed of woody tissue, such as we do not meet with in any but 

 the highest Cryptogamia, yet the special office of this is to afford mechani- 

 cal 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, 

 not only the pith and the bark, with the ' medullary rays,' which serve 

 to connect them, but that ( cambium-layer' intervening between the bark 

 and the wood ( 372), 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, where ^QV 'growth is taking place; as, for ex- 

 ample, in the 'spongioles' or 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 develop- 

 ment, that woody structure is found in them, its function (as in the 

 stem) being merely to give support to their softer textures; and the small 

 proportion of their substance which it forms, being at once seen in those 

 beautiful ' skeletons,' which, by a little skill and perseverance, may be 

 made of leaves, flowers, and certain fruits. All the softer and more 

 pulpy tissue of these organs is composed of cells, more or less compactly 

 aggregated together, and having forms that approximate more or less 

 closely to the globular or ovoidal, which may be considered as their origi- 

 nal type. 



351. As a general rule, the rounded shape is preserved only when the 

 cells are but loosely aggregated, as in the parenchymatous (or pulpy) 

 substance of leaves (Fig. 235), and it is then only that the distinctness of 

 their walls becomes evident. When the tissue becomes more solid, the 

 sides of the vesicles are pressed against each other, so as to flatten them 

 and to bring them into close apposition; and they then adhere to one an- 

 other in such a manner, that the partitions appear, except when carefully 

 examined, to be single instead of double as they really are. Frequently 

 it happens that the pressure is exerted more in one direction than in an- 

 other, so that the form presented by the outline of the cell varies accord- 

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