ELONGATION AND INTERNAL DIFFERENTIATION 297 



breaking the continuity, and so long as this continuity is maintained, the differ- 

 ence in growth may be accounted for by mere tension. 



These tensions have been termed tissue tensions, and they deserve a word 

 of explanation at this point. If we extract, by means of a cork-borer, the medulla 

 of a young internode of the sunflower, we may easily observe that the isolated 

 medulla increases in length by a certain percentage, while the peripheral region 

 shortens also to a certain extent. In the uninjured stem the cortical region 

 is in a state of extension while the medulla is in a state of compression. If we 

 split the stem longitudinally the two halves bend outwards like bows, so that 

 the cortex and the medulla assume their proper lengths, the former contracting, 

 the latter expanding. If we strip off a ring of cortex from a branch and sever 

 it at one point and then attempt to put it back again over the wood, we shall 

 find that the ring is too short; it has contracted tangentially. Transverse 

 tension, therefore, exists as well as longitudinal. These phenomena of tissue- 

 tension have been studied with great care, because it was expected that con- 

 clusions as to various physiological phenomena might be obtained from them. 

 These expectations have not been fulfilled, and hence the brevity of our refer- 

 ence to the subject. When we come to the consideration of the phenomena of 

 movement we will take an opportunity of again referring to the matter. 



After this digression let us return to the subject of tissue-differentiation. 

 In addition to the form of the cell, the peculiarities of its membrane are of 

 importance. Just as from a chemical point of view we may distinguish a whole 

 series of alterations in the cell-wall, so from the physical standpoint the cell- 

 membrane may exhibit varied modifications. Lastly, there are the morphologi- 

 cal differences occasioned by the degree to which the membrane becomes 

 generally or partially thickened. 



Thirdly, there are all the cell-contents to be taken into account. The 

 presence or absence of vacuoles, nuclei, chloroplasts, leucoplasts, starch-grains, 

 and other constituents, are all characters of the several types of cell. In many 

 cells contents are absent altogether, that is, the living contents are replaced by 

 water and air ; nevertheless these constituent cells may also carry out im- 

 portant functions in the plant. 



Lastly, we must note that certain elements, when fully formed, may unite 

 with other neighbouring elements of similar form by complete or partial absorp- 

 tion of their transverse walls. Such cell- fusions stand in marked contrast to 

 single cells, but this contrast is neutralized by the fact that the independence 

 of individual cells, notwithstanding the sliding growth previously referred to, 

 is by no means complete. For all living cells are connected to each other 

 by delicate strands of protoplasm which pierce the cell-membranes, and on the 

 basis of this discovery we may affirm that in the complex plant, whose body is 

 broken up by numberless cell-walls, only one protoplasmic body is to be found, 

 just as in Caulerpa or Mucor. 



Bibliography to Lecture XXIII. 



ASKENASY. 1874. Bot. Ztg. 32, 237. 



ASKENASY. 1878. Verb, naturw. Verein Heidelberg, 2, i. 

 ASKENASY. 1879. Ibid. 2. 

 ASKENASY. 1890. Ber. d. bot. Gesell. 8, 61. 

 BARANETZKY. 1879. Mem. Acad. Petersbourg, VII, 27, No. 2. 

 BERTHOLD. 1882. Jahrb. f. wiss. Bot. 13, 607. 

 [BERTHOLD. 1904. Phys. d. pflz. Organisation. Leipzig. 2, 146.] 

 BREFELD. 1877. Unters. aus d. Gesamtgebiete d. Mycologie, 3, 61. 

 BUCHNER. 1901. Zuwachsgrosse u. Wachstumsgeschwindigkeiten bei Pflanzen. 

 Leipzig. 



