424 



PHANEROGAMS. 



female flowers, and last of all in the dioecious condition of the plants them- 

 selves \ 



The fertilised embryonic vesicle of Phanerogams is not directly developed into 

 the embryo ; it first of all produces a pro-embryo, the Suspensor, — growing towards 

 the base of the embryo-sac and dividing, — which we have already met with in 

 Selaginella, and on the apex of which arises a mass of tissue at first almost 

 globular, and from which the embryo is developed. The development of the embryo 

 usually proceeds, even before the maturity of the seed, to such an extent that the 

 first leaves, the primary axis, and the first root, can be clearly distinguished. It is 

 only in parasites and saprophytes devoid of chlorophyll that the embryo usually 

 remains rudimentary until the dissemination of the seeds without discernible external 

 differentiation; while in those Phanerogams which contain chlorophyll the embryo 

 not unfrequently attains a very considerable size and external differentiation (as in 

 Pinus, Zea, Aesculus, Quercus, Fagus, Phaseolus, &c.) Independently of any curv- 

 ing of the embryo, the primary apex of its stem always lies originally pointing 

 towards the bottom of the embryo-sac (the base of the ovule) ; the first root 

 (primary root) coincides with a posterior prolongation of the primary stem ; it 

 faces the apex (micropylar end) of the embryo-sac, and is of distinctly endogenous 

 origin, inasmuch as its first rudiment at the posterior end of the embryo is covered 

 by the nearest cells of the pro-embryo. 



The Apical Cell of the punctum vegetatiojiis, which is easily recognized in many 

 Algae, in Characese, Muscinese, Ferns, Equisetaceae, and Rhizocarpeae, as the primary 

 mother-cell of the tissue, has already, as we have seen, lost its significance in the 

 Lycopodiaceae. The apical growth of the axes, leaves, and roots of Phanerogams can 

 no longer be referred to the activity of a single apical cell from which the whole 

 primary meristem has proceeded. Even in those cases where a single cell (not, 

 however, of preponderating size) occupies the apex, and the arrangement of the 

 superficial cells of the punctum vegetationis appears to point to it as the primary 

 mother-cell, it is nevertheless by no means to be assumed that all the cells, and 

 especially the internal mass of the primary meristem, has proceeded from it. The 

 primary meristem of the punctum vegetationis consists of a large number of usually 

 very small cells, more or less evidently disposed in concentric layers ; an outer simple 

 layer, the dermatogen, may be recognized in Angiosperms as the immediate continu- 

 ation of the epidermis of the older parts, and is continuous even over the apex of 

 the punctum vegetatio7iis. Beneath it lies a second layer of tissue (the periblem), 

 consisting usually of a few layers of cells, which covers the apex and passes lower 

 down into the cortex ; this envelopes a third inner mass of tissue (the plerome) 

 terminating beneath the apex as a single celP (Hippuris, &c.) or as a group of cells ; 

 and out of it proceeds either an axial fibro-vascular body (in the roots and stems 

 of water-plants), or the descending arm of the fibro-vascular bundles. In harmony 

 with this the root-cap does not proceed, as in Cryptogams, from transverse divisions 



^ Compare what is said on Dichogamy in Book III. 



^ As in so many other respects, here also Isoetes shows an affinity to Phanerogams, as is 

 evident from Nageli and Schwendener's researches on the apical growth of roots. (Compare Niigeli's 

 Beitragen, 1867, Heft IV, p. 136.) 



