THE PRIMARY MERISTEM AND THE APICAL CELL. 



125 



outer ones form a peripheral layer, and the six inner ones a central body. The 

 longitudinal section, Fig. 102, A, shows this wall at c c, and it may thus be seen how 

 the mass of the substance of the root is broken up by it into an outer layer c and 

 an inner thick bundle c c c c. Out of the former arises by further division a tissue 

 which becomes differentiated further backwards into epidermis and cortex (between 

 and c) ; the axial bundle c c c c, o'a the other hand, which is the result of further 

 longitudinal divisions of the inner sections of the sextants, forms the procambium- 

 cylinder of the root, in which arise the vascular bundles. In this case also the 

 first separation of the subsequent masses of tissue is occasioned by the first divi- 

 sions of the youngest segments; but a comparison of the corresponding process 

 in the stem of Equisetum shows that the mass of tissue which is formed from the 

 central portions of the sextant, has quite a diff"erent signification ; and the same is 

 the case with the peripheral layer. A further insight into the origin of the forms of 

 tissue of the root out of these portions of the primary meristem will be afforded in 

 the consideration of Ferns and Equisetaceas. 



In conclusion, it may be remarked that the segments of the apical cell, where 

 they arise in two or three rows, have at first a position oblique to the ideal axis of the 

 organ, and enclose an angle open towards the apical cell ; but, in consequence of 

 growth, the position of the segments generally changes so that they come to lie 

 gradually more transversely, and finally at a certain distance from the apical cell 

 the principal walls lie at right angles to the axis of the organ. The process is not 

 clearly shown in Figs. loi and 102 ; but more evidently in examples to be brought 

 forward later {e.g. Fig. 142). 



(b) Pimcta Vegctaiionis ivilhout an Apical Cell occur generally in Phanero- 

 gams. The apical region of growing shoots, leaves, and roots consists of a 

 primary meristem, the cells of which are very small in proportion to the circum- 

 ference of the whole piinctiim vcgctationis^ and very numerous. It has not yet been 

 demonstrated whether only the cells next the apex can be traced back to a single 

 primary mother-cell, although sometimes undoubtedly one cell lying at the apex is 

 distinguished by somewhat greater size and by its figure. In many shoots the 

 surface of the apex seen from above shows an arrangement of the superficial rows 

 of cells which to a certain extent points to this one cell as their common primary 

 mother-cell ; but even if this were the case, which is by no means proved, it is, 

 on the other hand, altogether impossible to connect genetically the inner layers of 

 cells also with this cell. The peculiar significance of the apical cell of Cryptogams 

 lies in the fact that all the cells of the primary meristem furnish evidence of different 

 degrees of descent from it. 



But as in Cryptogams certain layers of the primary meristem are prepared by 

 the first divisions of the segment-cells to pass over into the differentiated tissue- 

 systems further backwards from the apex, so also in Phanerogams a definite 

 arrangement of the cells is brought about in the primary meristem of the punctum 

 vegetationis of such a kind that the single layers of the primary meristem, when 

 followed further backwards, have a genetic relation with the epidermal tissue, the 

 cortex, and the fibro-vascular bundles, and may be recognised as the first rudiments 

 of them. The outermost layers run uninterruptedly over the apex of the punctum 

 vegelationis, overarching an inner mass of tissue of the primary meristem, which 



