THE PRIMARY MERISTEM AND TUE APICAL CELL, 11/ 



Sect. 19. The Primary Meristem and the Apical Cell ^ — At the growing 

 ends of shoots, leaves, and roots, the forms of cell-tissue hitherto described do not 

 yet exist ; here is found a uniform tissue, the cells of which are all capable of 

 division, rich in protoplasm, with thin and smooth walls, and containing no coarse 

 granules. This tissue is termed Primary IMeristem ; it is a meristem because all 

 the cells are capable of division, and must be considered primary (rather, perhaps, 

 proto-meristem) because it presents the primary condition of the tissue, out of 

 which the different forms of the permanent tissue are successively formed by 

 differentiation. If the structure of the plant is in general simple, as in Algae and 

 Characeje, the cell-forms arising from the primary meristem only differ slightly 

 from one another. If the plant belongs to a higher type, as in Vascular Cryp- 

 togams and Phanerogams, from the uniform undifferentiated primary meristem 

 proceeding from the growing apex layers of tissue of a different character first 

 originate, within which, by further development of their cells (at a still greater 

 distance from the primary meristem), the different cell-forms of the epidermal 

 and fundamental tissue, as well as of the fibro-vascular bundles, finally arise. 

 The differentiation takes place so gradually, and at such a different time in the 

 various layers of the tissue, that no definite limitation of the primary meristem 

 proceeding from the apex is possible. As growth proceeds at the end of shoots, 

 leaves, and roots, portions of the primary meristem become gradually transformed 

 further backwards into permanent tissue ; but the primary meristem is always again 

 renewed by the production of new cells close to the apex. Nevertheless whole 

 organs, the apical growth of which soon ceases, may at first consist entirely of 

 primary meristem, which finally passes over altogether into permanent tissue, so 

 that no primary meristem is left. Examples of ihis are furnished by the deve- 

 lopment of the fruit of Mosses, of the sporangia of Ferns, and even of most 

 leaves and fruits of Phanerogams. 



The terminal portion of an organ with permanent apical growth, consisting 

 entirely of primary meristem, is termed the Pimcium Vegetaiionis ; not unfrequently 

 (but by no means always) it projects as a conical elongation, and is in this case 

 distinguished as the Vegetative Cone. 



The production and renewal of the primary meristem commence with the 

 cells lying at the apex of the puncium vegciationis ; and, by the manner in which 

 this happens, two extreme cases may be distinguished, which are however united by 

 transitional forms. In the one case, the usual one with Cryptogams though not 

 without exception, the whole of the cells of the primary meristem trace their origin 

 back to a single mother-cell, lying at the apex of the punctum vegetationis and 



* Nageli, Die neueren Algensysteme. Neueuburg 1S47. — Cramer in Pflanzenphysiol. Unter- 

 suchungen, Heft III. p. 21. Zurich. — Pringsheim, Jahrb. fiir wissen. Bot. III. p. 484. — Kny, ditto, IV. 

 p. 64.— lianstein, ditto, IV. p. 238.— Geyler, ditto, IV. p. 481.— Miiller, ditto, V. p. 247.— Rees, 

 ditto, VI. p. 209.— N.-lgeli und Leitgeb, in Beitriige zur wissen. Bot. Heft IV. Miinchen 1867.— 

 J. Hanstein, Die Scheitehellgruppe im Vegetationspunkt der Phanerogamen (in the Festschrift der 

 niederrh. Ges. fiir Natur- und Heilkunde. Bonn, und Monatsiibersicht of the same Society, July 5, 

 1S69).— Hofmeister, Bot. Zeitg. p. 441, 1870.— Leitgeb, Sitzungsb. der Wiener Akad. 1868 and 1869, 

 and Bot. Zeitg. nos. 3 and 34, 1871.— Reinke in Hanstein's Botan. Untersuchungen, Heft III. 

 Bonn 1871. 



