ELONGATION AND INTERNAL DIFFERENTIATION 



289 



find the maximum elongation finally taking place in zone I. That the 

 graphic representation figured at Fig. 85 agrees in this important point of 

 the shifting of the zone of maximum growth will be seen by comparing it with 

 Sachs's measurements (1873), some of which we may quote : — 



It is obvious, however, that this forward march of the zone of maximum 

 growth towards the apex is apparent only ; for our curves show in the clearest 

 possible way that the maximum zone is always situated at approximately the 

 same distance from the apex, and if we carry out our observations at short 

 intervals and always mark afresh each time, we shall find that this will be clearly 

 proved by the measurements also. We thus 

 see that to allow longer time to elapse between 

 each pair of observations in determining the 

 region of maximum growth would lead to serious 

 error. 



By way of summary we may say : The grow- 

 ing region of the root is limited to a few millimetres 

 behind the growing point. In the course of this 

 growing region each individual transverse zone 

 passes through a grand period, those zones which 

 are nearest to the apex are at the beginning, those 

 farthest away from it at the end of their grand 

 periods. 



The shortness of the growing region in the 

 root is a matter of great importance to it. The 

 root in its passage into the soil has to overcome 

 great opposition, and we may compare it (Sachs, 

 1873, 424), to a nail driven into a plank of 

 wood. As in the case of the nail so in the root 

 there is a danger of bending : the shorter the 

 growing point the more safely it enters the soil. 

 If we contemplate this entry of the root into the soil, the pointed form of the 

 growing point covered by its cap is explained, and we further appreciate the 

 reason why the lateral roots develop first at some distance behind the apex, and 

 from parts which have long before attained their maximum growth and have 

 now become quiescent. If the new roots arose from the growing point itself 

 they would add to the difficulties the root has in entering the soil, or they 

 would have to form a kind of bud — just as, in fact, we see stem buds, e. g. of 

 seedlings, boring through the soil. 



Roots which do not live in soil, especially the long aerial roots of lianes 

 and epiphytes, exhibit a much longer growth zone, as Sachs (1873) long ago 

 pointed out, and as Went (1895) has more recently confirmed. Thus Went 

 found a growing zone 40 mm. long in Philodendron. These aerial are, in fact, 

 comparable with shoots, growth in which we will study by and by. 



Let us now inquire what the total growth resulting from the addition of 

 the several increases of the individual zones amounts to, as evidenced by the 

 forward development of the root apex in space. We assumed above that growth 

 was uniform, and, acting on that assumption, we have represented the undermost 

 of our curves (Fig. 85) by a straight line. Askenasy has, indeed, shown that 



Fig. 85. For explanation see Text. 



