ELONGATION AND INTERNAL DIFFERENTIATION 293 



at once passes into a state of rest. This is the case in many lianes, where speci- 

 ally formed apices fulfilling particular functions (' forerunner-tips,' Raciborski, 

 1900) are produced long before the rest of the lamina is completed. The elon- 

 gation is basipetal also in the long leaves of Monocotyledons, owing to the 

 development of an intercalary growing zone at their bases. The distribution 

 of growth in this case is illustrated by the following numbers, which represent 

 the fortnightly increments in zones, each 2.5 mm. long, marked off on the 

 leaf of the onion (Stebler, 1878) : — 



Intercalary growth zones are of frequent occurrence in leaves ; but it is 

 impossible for us to enter into a discussion of the effect such intercalary growth 

 zones have on the formation of the leaf ; reference must be made to the morpho- 

 logical literature, and more especially to Goebel (1898-1901). 



It was noted earlier that leaves during their embryonic growth assumed 

 special formations, which were explicable partly mechanically, partly biologically. 

 If, taking a simple case, the leaf, owing to excessive growth on the under side, 

 protects its growing point by bending over on itself, it is obvious that this 

 curvature must be again compensated for during elongation by increased growth 

 on the upper side. Unequal growth in length of this kind is to be found not 

 only in cases where it is necessary to compensate for previous curvatures or 

 foldings, but it occurs also by no means infrequently in uncurved rudiments, 

 transforming them from a straight embryonic form to a permanently curved 

 adult form. The physiologist sees, more often, indeed, than he desires, how 

 the root-apex or the tip of the shoot pushes itself forward, not in a straight line 

 but in a curve, for such curvatures, known as nutations, resulting from small 

 irregularities in growth of the different sides, frequently interrupt experirnental 

 work to a serious degree. We shall take another opportunity of referring to 

 such cases. 



Only a few examples of specially rapid growth need be quoted, for the 

 measurements which have been noted vary extremely. The following table 

 gives the maximum rate of growth per minute for a few plants : — 



Didyophora (Moller, 1895) 5 mm. 



Stamens of Gramineae (AsKENASY, 1879) i-8 ,, 

 Bambusa (Kraus, 1895) 0-4 ,, 



Coprinus (Brefeld, 1877) 0-225 » 



Botryiis (Reinhardt, 1892) 0-034 ,, 



There are cases known where it is possible to watch the organ actually 

 growing without employing a microscope. These observations have, however, 

 no scientific value, since the actual rate of growth, i. e. the increase in unit of 

 length per unit of time cannot be expressed. The growing region in Bambusa 

 is very long (several centimetres) ; in Botrytis it is only o-02 mm. in length, 

 for although the former shows ten times as great an increase as the latter per 

 minute, still its rate of growth is much less. To work out the rate of growth 

 we must use percentages. The following table gives the increase per cent, of 

 the growing zone per minute (BIjchner, 1901) : — 



Pollen-tube of Impatiens hawkeri 



,, ,, balsantina 



Hypha of Mucor stolonifer 

 Botrytis 



Stamens of Gramineae 

 Shoot of Bambusa 



,, Bryonia 



