138 MORPHOLOGY OF TISSUES. 



exist; here is found a homogenous 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 Men's iem ; it is a meristem because all 

 the cells are capable of division, and primary because it presents the condition out 

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

 tiation (' Proto-meristem ' might perhaps, therefore, be a better term). If the general 

 structure of the plant is simple, as in Algae and Characeae, 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 Cryptogams and Phanerogams, layers of tissue of 

 a different character first originate from the undifferentiated primary meristem which 

 proceeds from the growing apex, and in these the different cell-forms of the 

 epidermal and fundamental tissues, as well as of the fibro-vascular bundles, finally 

 arise by further development, at a still greater distance from the primary meristem. 

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

 various layers of the tissue, that it is impossible at any one time to assign a definite 

 lower limit to the primary meristem. As growth proceeds at the end of shoots, leaves, 

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

 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 this are furnished by the development of the 

 sporogonium of Muscineae, 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 Growing Point or ' Punctum Vege- 

 tationis;' not unfrequently (but by no means always) it* projects as a conical 

 elongation, and is in this case distinguished as the Vegetative Cone or Cone of 

 Growth. 



The production and renewal of the primary meristem commence with the 

 cells lying at the apex of the growing point ; 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 growing point and called 

 the Apical Cell. In some Cryptogams, on the other hand, and in Phanerogams, 

 there is no single apical cell of this character; even when a cell lies at the 



ditto, vol. IV. p. 64. — Hanstein, ditto, vol. IV. p. 238. — Geyler, ditto, vol, IV. p. 481. — Miiller, 

 ditto, vol. V. p. 247. — Rees, ditto, vol. VI. p. 209. — Nageli und Leitgeb, in Beitrage zur wissen. 

 Bot., Heft IV. Miinchen 1867. — J. Hanstein, Die Scheitelzellgruppe im Vegetationspunkt der 

 Phanerogamen (in the Festschrift der niederrh. Ges. fUr Natur- und Heilkunde, Bonn ; and 

 Monatsiibersicht of the same Society, July 5, 1869). — Hofmeister, Bot. Zeitg. 1870, p. 441. — 

 Leitgeb, Sitzungsb. der Wiener Akad. 1868 and 1869, and Bot. Zeitg. 1871, nos. 3 and 34. — 

 Reinke in Hanstein's Botan. Untersuchungen, Bonn 1871, Heft III. — Russow, Vergleich. Untersuch. 

 der Leitbiindelkryptogamen, in Mem. de I'Acad. Imp. de St. Petersbourg, 7th ser., vol. XIX. no. i, 

 Petersburg 1872. — Warming, Recherches sur la ramification des Phanerogames, in Vidensk. Selsk., 

 Skr. 5, Rakke 10, B. i, Copenhagen 1872 (Danish, with French abstract). 



