440 PART III. THE CLASSIFICATION OF PLANTS. 



developes tubular outgrowths, whicli penetrate into and destroy 

 the tissue of the integument (e.g. Rhinanthus, Lathrsea, some 

 Labiatae). In some cases, however (e.g. Grymnosperms, Scitamineae, 

 most Nymphseaceae, Piper), the macrospore does not grow to such 

 an extent, so that a considerable mass of nucellar tissue is left, 

 which persists to some extent in the seed as perisperm, its cells 

 being then filled with nutritive substances. This may be due, as 

 in the Gymnosperms, to the fact that the macrospore is covered, 

 towards the micropyle, by a mass of nucellar tissue formed by 

 the growth and repeated division, both periclinal and anticlinal, 

 of either the tapetal cell, or of the apical epidermal cells of the 

 ovule, or of both ; or, as in the other cases, to the fact that the 

 macrospore, in its growth, which is relatively slight, does not 

 absorb the chalazal portion of the nucellar tissue. 



General Histology. The following are the principal characteris- 

 tic features : — The apical growth of shoot and root is only excep- 

 tionally effected by means of a single apical cell : the small-celled 

 meristem of the growing-point of the stem is more or less distinctly 

 differentiated into dermatogen, periblem, and plerome, so that the 

 stem has a true epidermis : the epiblema of the root is either the 

 persistent innermost layer of the original many-layered endodermis 

 (most Dicotyledons, Gymnosperms), or it is the external layer of 

 the cortex (Monocotyledons ; see p. 154) : stem and root are mono- 

 stelic, with but few exceptions (p. 152) : the vascular bundles of 

 the stem are generally collateral : both root and stem generally 

 present secondary growth in thickness (except Monocotyledons, 

 and a few other cases) by means of a normal cambium-ring (for 

 abnormal cases, see p. 204) : the growing-points of the lateral roots 

 are developed from the pericycle of the parent root (see p. 186). 



The Emhryogeny of the Sporophyte. The sporophyte is developed 

 from the fertilised oosphere in the ovule. The development of the 

 embryo is not continuous, but is in two stages, which may be con- 

 veniently distinguished as the intra- seminal and the extra-seminal. 

 The intra-seminal stage includes the whole of the development 

 which the embryo undergoes during the conversion of the ovule 

 into the ripe seed — that is, during what is known as the " ripening 

 of the seed." The extra-seminal stage includes the development 

 of the embryo which follows the sowing of the seed ; — that is, the 

 escape of the embryo from the seed, and the gradual development 

 of the characters of the adult plant. The interval between these 

 two stages may be brief, or it may extend over many years if the 



