GROWTH, IRRITABILITY AND MOVEMENT 



143 



points of root and shoot is a region of cell-elongation, which is also 

 the region in which the extension of the organ itself chiefly occurs. 

 These growth zones can be located by marking on a young shoot or a 

 root a series of initially equidistant dots of Indian ink (Figs. 88, 89). 

 After a day or so the distances between the dots are re-measured, 

 and the amount of elonga- 

 tive growth in this way is 

 ascertained for different 

 parts Of the organ. In the 

 shoot the greatest elonga- 

 tion w r ill be shown in a region 

 considerably below the tip, 

 w T hile it gradually diminishes, 

 on the one hand towards the 

 tip and on the other down- 

 wards, until a point is 

 reached where growth has 

 ceased (Fig. 88). This de- 

 monstration at once accounts 

 for the general contour shown 

 by growing shoots. In the 

 apical bud the leaves are 

 closely grouped because the 

 axis has not yet extended 

 to its full dimensions. The 

 length of the internodes (or 

 intervals between the leaves) 

 increases downwards till their 

 growth is complete, and 

 their full length has been 

 attained. Below that point 

 the stem has become rigid, 

 owing to general thickening 

 of the cell-walls as they ma- 

 ture. In roots the cells enter 

 on the phase of elongation with less delay and complete it more 

 rapidly than in stems : with the result that the region of most active 

 growth is more compact. It is localised within a few millimetres of the 

 tip of the root (see Fig. 89, also p. 88). This proximity of the zone of 

 growth to the apex may facilitate the penetration of the soil by the 

 root : if it were further back, bending of the apical part might follow, 



Fig. 88. 



The left-hand figure shows a growing shoot at the 

 beginning, the right hand-figure at the end of the period 

 of observation. See Text. (After Errera.) 



