ARRANGEMENT OF LEAVES. 



27 



due to a torsion or gyratory movement made by the shoot during its 

 growth ; in those cases where the grow r th is rapid, as the " leader 

 shoot " of species of Abies and Picea, notably Abies Nordmanniana and 

 A. Veitcliii, Picea sitchensis and others, this gyratory movement may be 

 detected by watching the progress of the shoot at frequent intervals. 

 Were the gyratory movement that gives rise to the spiral arrangement 

 absent, it is evident that the leaves would then be in straight lines, 

 an arrangement unquestionably detrimental to the well-being of the 

 plant since the area of leaf surface exposed to direct sunlight would 

 be enormously reduced. 



Fig. 12. Branchlet of Tsuga Mertensiana with leaves spreading in various directions. 



The decussate arrangement that is to say, when each pair of 

 opposite leaves is placed at a right angle to the pair immediately 

 above and below it is common throughout the Cupressineae except in 

 those species of Juniper in which subulate (awl-shaped) adult leaves 

 predominate (Jumperus communi*, J. rigida, J. drupacea, J. oxyccdrus, 

 etc.), and these are always in whorls of three each. The whorled 

 arrangement occurs only in Juniper us and in the phylloid growths 

 (cladodes) of Sciadopitys.* 



In all the Firs and in many Pines the leaves are inserted on out- 

 growths of the suberous or corky layers of the bark called pulvini 

 (pulvimis, a cushion), and these pnlvini are prolonged into spiral ridges 



* The arrangement of leaves, whether foliage, floral or carpellaiy (fruit bearing), on the 

 axial structures that produce them is described in Botany under the name ot Phyllotaxis. 

 The subject is treated of in all the best text books. 



