294 METAMORPHOSIS 



We may also calculate the time necessary for the attainment of a certain 

 increase, e. g. for doubling the length of the organ : 



Botrytis i rain. 



Bacteria 20-30 ,, 



Grass stamen 2-3 



Root of Viciafaba about 1 80 ,, 



If we know the duration of growth we may from the rate of growth and the 

 extent of the growing zone calculate the definite amount of elongation a plant 

 part undergoes. According to the variations in these factors the size of the 

 plant is determined, and it is, as every one knows, dependent also on external 

 factors in manifold ways, and yet in each case it is specifically different. Draba 

 verna in the course of its vegetative period attains the dimensions of a few centi- 

 metres, Ricinus or Helianthus must be measured in metres ; Calluna vulgaris, 

 after ten years, still remains a small shrub, but a Eucalyptus tree reaches the 

 height of Strassburg Cathedral (compare p. 62). A definite size is as much 

 a specific characteristic of an organism as leaf -arrangement is ; the entire 

 organization of a plant is co-ordinated with the attainment of a certain size. 

 This is a point which SACHS (1893) has demonstrated most clearly, showing 

 what an impossible monstrosity would result were a Marchantia enlarged fifty- 

 fold, or diminished to a like extent. 



We have as yet limited ourselves to a consideration of the longitudinal 

 extension of the parts mapped out at the growing point. [BERTHOLD (1904) has, 

 as already remarked on p. 284, advanced ' stretching ' as a characteristic phase of 

 growth in addition to elongation. He understands by that term the ' inflation ' 

 which many parenchymatous cells of the leaf, the root, or the stem-cortex under- 

 go after the whole organ has reached its maximum growth in length. Whether 

 this ' stretching ' is identical or not with the primary increase in thickness about 

 to be described it is impossible for us to judge.] Every microscopic investiga- 

 tion, however, demonstrates the fact that increase in thickness also takes place. 

 The diameter of the fully-formed root or stem is greater, and often markedly 

 so, than that of the region just behind the growing point. This is demonstrated 

 by Fig. 76, which shows an increase in the diameter downwards in the periclinal 

 cell rows. Growth in thickness has been much less carefully studied than 

 growth in length, still all the essential features which we have learned to recog- 

 nize in growth in length are also found here. In the first place we can establish 

 the existence of a 'grand period'. On account of anatomical relations we 

 distinguish * primary ' and ' secondary ' growth in thickness. Primary growth 

 in thickness is universally distributed, and consists in the increase in size of 

 all cells, which at first divide, but which cease to do so later on. Not infre- 

 quently primary growth in thickness begins vigorously just after growth in 

 length ceases, and FRANK (1892) has established the fact that an internode of 

 the sunflower which has reached its greatest length may increase in diameter 

 until it has become nearly five times its original size. Many plant-organs which 

 exhibit considerable dimensions in the transverse direction, e. g. fruits, tubers, 

 &c., must attain their permanent form by such primary growth. In certain 

 groups, especially in Dicotyledons and Gymnosperms, we find another mode of 

 increasing in thickness which we may term secondary, and by which a con- 

 tinuous growth is effected in stem and root for many, even hundreds, of years. 

 The difference between primary and secondary increase in thickness does not 

 lie in duration however, for in palms (and also tree-ferns) primary increase 

 in length goes on for many years; the characteristic feature of secondary 

 increase in thickness is the existence of a special growing layer, an inter- 

 calary zone known as cambium. Cambium, in so far as it lies within the 

 vascular bundle, is formed from a tissue which remains over, after the forma- 

 tion of the bundle, between the xylem and the phloem regions, which does 



