MOVEMENTS OF GROWTH. 375 



cover them with a bell-glass. At the end of twenty-four hours 

 we find that, whereas no growth is to be observed in the upper 

 half, growth has taken place in the lower half. In Fig, 121 is 

 depicted the lower half of an internode of Secale after it has 

 been kept for twenty-four hours in moist air. K is the node. 

 The part a has been thrust by growth above the outer parts. 



In the bean (Phaseolus), and in many other plants, the region 

 of the internodes which is in a state of elongation, is situated at 

 their summit. If at the upper end of the second internode of a 

 bean, i.e. the segment of the stem which follows the epicotyl, we 

 make two marks with Indian ink some distance from one another, 

 while the third internode is already in a state of active elongation, 

 it will be found at the end of twenty-four hours that the distance 

 between the marks has considerably increased. The bean inter- 

 nodes are still growing at their upper ends after their lower ends 

 have ceased to grow. Their behaviour is thus quite different 

 from that of the internodes of grasses. 



1 See Sachs, Arbeiten des botan. Irutituts in Wurzburg, Bd. 2. 



2 See Errera, Ber. d. Deutsehen botan. Gesellschaft, 1886, p.. 441, 



3 Berthold, Studien ilber Protoplasmamechanik, 1886, p. 219. 



* It may be maintained, from the results of the recent researches of 

 mann, Correns, Zacharias, Klebs, and others, that under normal conditions 

 the surface growth of the cell membranes is brought about by apposition and 

 intussusception. The former leads to the on-laying of fresh layers of cell 

 material, the latter to the in-laying of fresh molecules of cellulose. The growth 

 of non-turgescent cells (see Untersuchungen aus d. botan. Institut zu Tubingen, 

 Bd. 2, p. 561) is effected by apposition alone ; the normal growth in thickness 

 of cell membranes (and also the growth -of starch grains-) likewise takes 

 place by apposition. 



152. Growth in Thickness. 



Growth in thickness proceeds by no means in one and the same 

 way in different plants and organs. We will here confine our- 

 selves to the growth in thickness of the stem structures and roots 

 of some dicotyledons. 



We take first for examination a shoot axis of Aristolochia Sipho, 

 3-4 mm. in thickness, employing either fresh material, or alcohol 

 material. In Fig. 122 is depicted the appearance of a delicate 

 transverse section under slight magnification. We easily make 

 out the general arrangement, and it may here be particularly 



OF THK 



UNIV-ERr-ITY 



