302 FOURTH GROUP. 



primary stem is at first always directed towards the base of the embryo-sac, that is 

 towards the base of the nucellus; the first or primary root coincides with the back- 

 ward prolongation of the primary stem, and is directed towards the apex or micropylar 

 end of the embryo-sac; it is of distinctly endogenous origin, its first rudiment at the 

 posterior extremity of the embryo being covered by the nearest cells of the suspensor. 



The apical cell of the growing point, which is easily recognised in many Algae, 

 in the Characeae, Muscineae, Ferns and Equisetaceae as the primary mother-cell of 

 the tissue, is replaced in the Lycopodiaceae, as we have seen, by a small-celled proto- 

 meristem. In the Phanerogams also the apex of the shoots, leaves and roots shows 

 no apical cell except for a brief period in the development of some embryos in the 

 Coniferae, but consists of a large number of usually very small cells rich in protoplasm 

 and with large nuclei, and with an arrangement in layers such as we find even in the 

 growing points of Vascular Cryptogams which have apical cells. But in the latter 

 the periclinal cell-walls, that is, the walls which run in the same direction with the 

 circumference, do not go quite up to the apex ; there is a gap always there in the cell- 

 system and this gap is occupied by the apical cell. It is usually the layering due to 

 the periclinal walls \vhich is most evident. An outer simple layer, the dermatogen, is 

 seen in the Angiosperms to be the immediate continuation of the epidermis of the 

 older parts, and extends without interruption over the apex of the growing point ; 

 beneath it lies a second tissue, the periblem, consisting usually of a few layers of cells 

 which is also continuous over the apex and passes behind into the cortex ; beneath 

 this again is an interior third mass of tissue, the plerome, which ends beneath the apex 

 in a single cell in Hippuris and others, or in a group of cells, and from which 

 proceeds either an axile strand of vascular bundles (the stems and roots of aquatic 

 plants), or the descending limbs of the vascular bundles. Hence the root-cap is not 

 formed, as in Cryptogams, from transverse segments of an apical cell, but in 

 Gymnosperms by repeated division in the direction of the apex and luxuriant growth 

 of the layers of the periblem of the root, in Angiosperms partly by a similar division 

 and growth in the dermatogen, partly in another way into which we must not enter 

 further here 1 . The first rudiments also of lateral formations, leaves, shoots and roots, 

 cannot be referred to a single cell in the Phanerogams in the same sense as in the 

 Cryptogams ; they are seen first as protuberances consisting of a few or more small 

 cells ; the protuberance, which is to become a shoot or leaf, shows from its first 

 appearance an inner mass of tissue which is in connection with the periblem of the 

 parent-structure, and is covered with a continuation of the young epidermis. 



The normal branching at the growing end of the shoots, leaves and roots is w-ith 

 few exceptions monopodial ; the generating axis as it grows produces lateral members 

 (shoots, lateral branchings of leaves, lateral roots) beneath its apex ; but many 

 inflorescences are formed by dichotomous branching, as in Valeriana. 



The monopodial branching of the axes of shoots is in radial organs usually 

 axillary, that is, the new shoots appear above the median line of very young, but not 

 always the youngest, leaves in the angle which they form with the parent-shoot. In 

 the Gymnosperms it is not usual for the axil of every leaf to produce a shoot; in the 

 Cycadeae for instance, as in many Filicineae, the branching of the stem is sometimes 



De Bary, Vergl. Anatomic, pp. 9-14, among other authorities, may be consulted on this point. 



