THE PHASE OF ELONGATION. 



423 



■geneous embryonic tissue into the various systems of tissues. Intercellular spaces 

 soon appear between the cells of the parenchyma ; the different kinds of cells 

 gradually obtain their characteristic shapes; the cells, at first only parts of the 

 whole in the growing-point, now become more individualised, peculiar processes of 

 growth making themselves evident in each one ; the size, external form, and the 

 differentiation of the contents of each cell are apparently independent of those 

 of the others — but of course only apparently. The various forms of tissues become 

 differentiated; the single cells 

 individualised. 



At the time when the 

 organs, or parts of organs, 

 have attained their external 

 form and definitive volume by 

 elongation, they are still imma- 

 ture — not yet fully developed 

 internally. It is true that 

 the differentiation of the forms 

 of tissues begins even in the 

 embryonic tissue of the grow- 

 ing-point, and continues during 

 the elongation, so that, at the 

 end of the latter process, the 

 epidermis, vascular bundles, 

 and the various parts of these, 

 as well as the different forms 

 of fundamental tissue, are 

 already to be plainly distin- 

 guished ; but the develop- 

 ment, especially of the cell- 

 walls, is not completed in this 

 state. 



(3) The third phase of 

 growth makes its appearance 

 as soon as the organs and their 

 parts have attained their per- 

 manent size and external form 

 by means of elongation, while, 

 however, still further changes 

 are taking place in the interior 



of the tissue. The lignification in the vascular bundles, the thickening of the 

 cell-walls, and the various kinds of pitting connected with it ; the development of 

 the sieve-tubes, the lignification of the sclerenchymatous elements; and more parti- 

 cularly the further development of the epidermis, especially the stomata, cuticularisa- 

 tion and perhaps silicification, and the development of hairs, now first attain 

 completion. The internal development of the tissues, which only occurs late and 

 after elongation is accomplished, is particularly striking in the case of the roots. 



FIG. 259.— Parenchyma cells from the median layer of the cortex of the root of 

 Fritaiaria impsrialis (longitudinal sections X sso). A very young cells situated 

 close above the apex of the root, still without cell-sap. B the same cells about 

 z mm. above the apex ; the cell sap J forms isolated drops (vacuoles) in the proto- 

 plasm /, plates of protoplasm lying between them. C the same cells about 

 7—8 mm. above the apex ; the two cells to the right below are seen from in front, 

 the large cell to the left below in optical section; the cell to the right above has 

 been opened by the section, and its nucleus is alTectod by the penetrating water 

 and exhibits a peculiar swollen appearance. 



