ELONGATION AND INTERNAL DIFFERENTIATION 291 



individual internode obviously goes through its own grand period, although we 

 know but little as to the distribution of growth in it. 



The case becomes more complicated if the shoot exhibits not merely growth 

 extensions of parts already laid down, but if it continually adds new parts to 

 those already present in the growing point, and if these also begin to elongate. 

 If the shoot be undivided into nodes and internodes as, for example, in Aspara- 

 gus, Linum, &c., growth may in general be considered as equivalent to that 

 seen in the root ; a single growth zone only is developed and in it a single 

 maximum. The only difference between such a case and the root is that the 

 growth zone is much longer. We are acquainted with shoots which have 

 growth zones 10 cm. or even 40-50 cm. in length, but in these cases the region 

 of maximum growth lies much further back from the apex than it does in 

 the root. 



As an example of a shoot with obvious segmentation and with a continu- 

 ously growing apex, we may select Char a or Nitella. These highly-organized 

 Algae increase by means of a terminal apical cell. Each segment of this apical 

 cell divides into two cells ; the upper cell is biconcave, and after several sub- 

 divisions becomes a node, the lower biconvex cell remains undivided and be- 

 comes an internode. The nodes retain approximately their original length, in 

 Nitella about 0-02 mm. (ASKENASY, 1878); the internode, on the contrary, 

 becomes stretched often as much as 2,000 times its original length. If we 

 correlate the length relationships of successive internodes on an actively grow- 

 ing shoot we obtain the following values (ASKENASY, 1878) : 



Internode 12345 678 



Length in mm. 0-02 0-07 0-16 0-45 3-33 14-0 33.5 35-0 



If we make the not improbable assumption that one internode undergoes 

 similar extensions in similar intervals of time, as shown above in the case of 

 successive internodes, then each internode will exhibit the following grand 

 period, where the increments in similar periods of time are indicated in mm. : 



0-05 0-09 0-39 2-88 10-77 X 9'5 I> 5 



In fact each individual internode in a segmented shoot passes through a 

 grand period independently. In each, also, a zone of maximum growth may be 

 demonstrated at some definite place, and possibly this shifts from the base 

 towards the apex (or in the reverse direction from the apex to the base), in the 

 same way as we have seen it do in the shoot of Picea. It not infrequently 

 happens that the region of the stem, where the zone of maximum growth occurs 

 last of all, exhibits not a simple stretching of the cells merely, but both cell 

 formation and cell elongation, lasting for a long time. In every individual 

 internode a portion of the primitive growing point remains, and this goes on 

 acting as an intercalary growing zone. In fact there is no line of demarcation 

 between localized extension and an intercalary growing zone. 



The question now arises, what is the total amount of growth resulting 

 from the activity of several independent growth zones? It is known that 

 often 3-4 or, in other cases, as many as fifty internodes are elongating at the 

 same time. The sum of their activities may give a single uniform curve of 

 growth not differing from that of the single internode of Fritillaria given above, 

 or it may be entirely different (ROTHERT, 1894). If only a few internodes be 

 concerned in the elongation, it might come about that a younger internode might 

 start growing after the older one had entirely or nearly ceased to grow, and thus 

 we should have a periodic rise and fall of the growth curve, that is to say 

 ' spasmodic variations ' such as we have previously drawn attention to. Such 

 variations are, generally speaking, to be found almost everywhere ; they owe 

 their origin, however, doubtless not to the cause just mentioned only. The 



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