GROWTH 475 



growth of the stem is the algebraical sum of the growth of 

 the iuternocles, many of which may be growing simultaneously. 

 They will consequently be at different parts of their grand period 

 at any particular moment. The region of growth in the stem is, 

 as a rule, nnich longer than that of the root. 



The growth of the leaf shows a little variation. The apical 

 groW' th, as a rule, is not very long continued, and the subsequent 

 enlargement of the leaf is due to an intercalary growing region 

 near the base. This area has the merismatic cells at about its 

 centre, and regions of greatest growth are on both sides of it. 

 This can be traced more easily in the elongated leaves of 

 Monocotyledons than in those of Dicotyledons. 



The grand period itself is not quite uniform, as the rates of 

 growth in the active region may and do vary with changes in 

 external conditions, and with differences in activity in the proto- 

 plasm from time to time. 



For the sake of simplicity of description it has been assumed 

 in the foregoing account that the turgidity of the cells in the 

 growing organ is uniform. This, however, is far from being 

 the case. There is generally a fairly regular variation in this 

 turgidity in the different parts of the growing organ. In the 

 simplest case we may consider one which shows a difference 

 in structure on two sides ; such a member is described as 

 dorsiventral. The two sides will often show a difference of 

 degree of turgidity, and consequently of rate of growth. If w^e 

 consider a leaf of the common Fern, we find that in its young 

 condition it is closely rolled up, the upper or ventral sm'face 

 being quite concealed. As it gets older, it gradually unfolds and 

 expands into the adult form. This is due to the fact that in 

 the young condition the tiurgidity and consequent growth are 

 greater on the dorsal side of the leaf, so that it becomes con- 

 volute. As it gets older the maximum growth changes to the 

 upper side, and so it becomes unfolded or expanded. These 

 two conditions are generally described under the names of lujiw- 

 nasty and ejpinasty respectively. 



These conditions are not confined to the leaves of ferns, but 

 may be detected in those of other plants, though to a less 

 degree. It is in consequence of them that the leaves of the bud 

 always fold over the apex of the stem from which they spring. 



Cylindrical organs may exhibit similar variations in growth. 

 One side of a stem may be more turgid than another, and the 

 maximum turgidity with its consequent growth may alternate 

 between two opposite sides. The increased turgidity of the cells 



