740 MECHANICAL LAWS OF GROWTH. 



Zone. Increment, 



ist 2*0 mm. 



2nd 2*5 .^^ 



3rd 4*5 



4th 6-5 



5th 5*5 



6th 3'o 



7th 1-8 



8th ro 



9th i-o 



loth o'5 



nth o"5 



1 2th o*5 



The maximum rapidity of growth lay therefore in the fourth zone, which was 

 originally situated at a distance of about io'5 mm. from the upper end of the inter- 

 node. 



As it is usual for several contiguous internodes of stems to be growing at the 

 same time, and the maximum rapidity of growth occurs, according to circumstances, 

 in the second, third, fourth, or fifth internode beneath the bud, the region of most 

 rapid growth is at a considerable distance from the apex of the stem, and especially 

 when the internodes attain a considerable length and several are growing at the same 

 time. In roots, on the other hand, the maximum rapidity of growth occurs much 

 nearer the punclum vegefatiom's, usually at a distance of only a few millimetres ; 

 and the portion of the root beneath its apex in which the chief part of the growth 

 takes place is consequently only a few millimetres long, while in stems with long 

 internodes it is often many centimetres in length. If therefore a root and stem 

 with long internodes are divided into zones of equal lengths, e.g. 1 mm., com- 

 mencing from the punctum vegeiaiiom's, the law of growth, as expressed by the 

 general formula given above, is the same in both cases, but with this difference, 

 that in the stem the number of zones that are increasing in length at the same time 

 is much greater than in the root, in consequence of the fact that in the last case 

 each zone completes its period of growth more quickly^; its curve is shorter and 

 more abrupt. 



Thus, for example, in a primary root of Vici'a Faha which grew in damp air 

 and which was divided, starting from the punclum vegeiatioftis, into zones each i mm. 

 in length, I found the following increments of growth in the first twenty-four hours 

 at a temperature of 20'5'' C. : — 



Zone. Increment. 



1 0th o*i mm. 



9th 02 



8th 0-3 



7th 0-5 



6th 1-3 , 



* It by no means however follows from this that the root grows more rapidly, t. e. attain? in the 

 same time a greater length than the stem. 



