368 PHYSIOLOGY OF GROWTH. 



When roots lie in water their total volume naturally increases, 

 notwithstanding the fact that they shorten ; and the volume 

 of their constituent cells likewise increases. Extension of the 

 cells takes place only in a direction at right angles to the axis of 

 the root ; the root becomes thicker, and we make the following 

 observations in order to determine this extension : 



We prepare transverse sections of the Carum roots '5 mm. in 

 thickness, isolate a median portion of each by two parallel cuts, 

 and indicate its length on paper under slight magnification (about 

 10 diameters) by means of a camera lucida. The sections are 

 now quickly put into water. If we again mark their length on 

 paper after about an hour, it is found on comparison that the 

 strips have increased in length. Dividing the lengths observed 

 by 10 (if we have worked with a magnification of 10 diameters), 

 we obtain an absolute value for the lengths of the strips before 

 and after the absorption of water. 



1 See H. de Vries, LandwirthschL Jahrbucher, Bd. 9, p. 37. 



149. Longitudinal Tension. 



To determine that in many plant structures longitudinal ten- 

 sions occur, straight internodes, or portions of such, are laid on 

 a piece of thick cardboard, on which fine lines have been drawn, 

 and the length of each is indicated by two marks made with a very 

 sharp lead pencil. Then with a sharp razor we remove strips 

 of the different tissues (epidermis, generally with the collenchyma 

 attached, cortex, wood, and pith, which we free from wood by 

 longitudinal cuts) the full length of the internodes, and without 

 dragging them, lay the isolated strips on the cardboard, and mark 

 off their length as before. We can now, by means of a millimetre 

 scale, determine the lengths of the uninjured internodes and 

 of the tissue isolated from them. We may employ for the 

 observations vigorously growing internodes, about 50 mm. long, 

 from Sambucus nigra, Nicotiana Tabacum, Vitis vinifera, or 

 Helianthus tuberosus. It is always found that the length of the 

 isolated strips of tissue increases from outside inwards, the pith 

 isolated from actively growing internodes being generally much 

 longer, and the isolated epidermis shorter, than the uninjured 

 internode, while a strip of tissue taken from the region between 

 the epidermis and pith is exactly, or nearly, the same length as 



