SECONDARY THICKENING. NORMAL DICOTYLEDONS. 477 



of the individual layers has been determined, giving the 'growth in mass;' while in 

 others the (radial) diameter, the breadth of ring has been measured, on the successive 

 dimensions of which the form of the stem depends. The growths in mass and 

 breadth of ring in a layer do not necessarily correspond one with another, because 

 the former may be greater in its lower portion, where its periphery is larger, though 

 the breadth of ring is smaller, than in its upper portion where the ring is broader. 

 The following rules may hold good as giving the consistent result of the published 

 investigations, which extend to the Oak, Beech, Alder, Silver Fir (Abies pectinata, 

 D. C), Scotch Fir (Pinus sylvestris L.), Red Fir (Abies excelsa Poir.), Larch, 

 Weymouth Pine (Pinus Strobus L.), &c. 



(i) In the shaft, i. e. in the unbranched stem between the basal ' stock ' and the 

 crown, the annual growth in mass increases in the case of trees which stand free, 

 from above downwards ; according to Nordlinger the average diameter of the ring 

 always increases simultaneously, while, according to R. Hartig, this may increase 

 or decrease, or remain the same, which agrees with the results of the radial measure- 

 ments made by Mohl on three trees grown in the open, which showed increase in 

 the upward direction. In closely planted trees, the average breadth of the rings 

 increases in the upward direction, according to Nordlinger, Mohl, and R. Hartig, 

 while according to R. Hartig's statement, which has been reasonably disputed by 

 Nordlinger, the grovt'th in surface or mass remains throughout approximately the 

 same. Trees, the crown of which has become stunted in consequence of their being 

 closely planted, show a diminution of the secondary growth in this direction, namely, 

 from above downwards, which may extend even to its complete absence in the lower 

 portion. 



(2) In the crown the growth increases in the downward direction, both in the 

 stem and branches. 



(3) In the basal stock a considerable increase in the secondary growth and the 

 average breadth of ring takes place in the older stems, i. e. in the outer layers ; this 

 starts from the upper side of the insertions of the roots, and may extend upwards to 

 f? different height (03-3 metres and more), according to the particular case. At 

 the points of insertion of vigorous lateral roots, the secondary growth is locally 

 increased in such a manner that the well-known projections of the stock, separated 

 by furrows, and in the case of tropical trees attaining huge dimensions, may arise in 

 the course of a few years. 



The 'forms of growth' mentioned under (i) are subject to change in the same 

 individual, when successively planted free, and in contact with others. For each 

 particular species of our forest trees one or the other form of growth is the rule, and 

 this depends on whether they usually grow in close contiguity throughout life, either 

 wild or in forest culture (e. g. Beech, Silver Fir, Red Fir), or whether in their later 

 years they become free (e. g. Scotch Fir, Larch, Oak, Alder) \ 



The dependence of the general form of the stem, whether it be more conical 

 or cylindrical, on the conditions mentioned is self-evident. In the same way it is 

 clear, that in the stems of exotic plants which deviate from the cylindrico-conical 

 form, as the spindle- or barrel-shaped stems of Bombaceae ^ the progression of the 



* Cor pare R. Hartig, Botan. Zeitg. 1870, p 513. 



^ See e.g. the figure of Bombax Mungiiba in Martins, Fl. Brasil. Tab. physiogn. X. 



