GROWTH. 355 



The grand period is exhibited as well by multicellular 

 organs as by single cells, and we shall best illustrate it 

 by reference to the former. We have seen (p. 338) that 

 a growing apex may be regarded as consisting of succes- 

 sive zones of cells : those nearest the apex consist of young 

 cells which are growing but slowly ; others, further from the 

 apex, consist of older cells which are growing rapidly ; others 

 again, still further from the apex, consist of nearly mature 

 cells which are ceasing to grow. This mode of regarding the 

 growing apex gives us an idea of the distribution of the 

 activity of growth in space. If, however, we regard these 

 zones, not as a series of successive zones, but as representing 

 successive stages in the growth of a single zone, we gain 

 a conception of the distribution of the activity of growth 

 in time, that is, of the Grand Period. As a matter of fact 

 any one zone does, in the course of its growth, occupy 

 the positions of these successive zones and exhibits the cor- 

 responding rates of growth. At first it lies near the apex 

 in the punctum vegetationis, and then its growth is slow : by 

 the formation of new cells in front of it, it is gradually re- 

 moved further and further from the apex, and its growth 

 becomes more and more rapid until the maximum rapidity is 

 attained ; as its distance from the apex continues to increase, 

 the rapidity of its growth rapidly diminishes, and ultimately 

 it ceases to grow. 



The following determinations of the grand period of growth of zones 

 of growing organs were made by Sachs. 



i. Zone i mm. in length, marked just behind the punctum vegeta- 

 tionis of a primary root of Vicia Faba. 



Time. Increment. 

 ist day (24 hours) r8 mm. 



2nd 37 



3rd 17*5 



4th 16-5 



5th 17-0 



6th 14-5 

 7th 7-0 



8th o-o 



232 



