IRRITABILITY 195 



and stronger mechanical tissues, and that this development 

 is a response to irritation. We have only to apply these 

 conclusions to the changes taking place in growing wood as 

 the season progresses to gain an idea as to one of the most 

 important influences contributing to the formation of "au- 

 tumn wood." 



Ordinarily only one ring is added to the wood each year. 

 Many woody plants, if defoliated by frost, caterpillars, etc., 

 so early in the season that growth has not ceased, will 

 open their latent buds, and develop a second set of leaves. 

 Under these conditions, with the sudden increase in the 

 demand for new conducting tissues, the young derivatives of 

 cambium will develop into elements resembling but not 

 quite equalling those of normal "spring wood." In this 

 way woody plants may form in nature two rings of wood in 

 a single year. It is claimed* that "spring" and "autumn" 

 wood may be found repeatedly alternating with one another 

 in a single season's growth in pine, if only during the grow- 

 ing season there are repeatedly alternating and sharply 

 contrasting rainy and dry periods. Pfeffer's caution f "that 

 an apparently similar result may sometimes be produced in 

 various ways" applies to this observation as well as to 

 experiments. 



The careful study of plants subject to the attack of gall- 

 insects and other pests should throw light on the relation 

 of the growth of wood to the demand made upon it. For 

 example, cross-sections of the younger branches of Monterey 

 Pine {Pinus radiata) which have been attacked by the leaf- 

 gaUing insect Diplosis pim-radiatEe,X show abnormalities in 

 the vascular tissues. Instead of the clearly marked annual 

 rings of wood, these branches have semi-annual rings which 

 correspond in size, position, and composition with the times 

 at which the plant is attacked by the gall-insect, at which 



* Lutz, K. G. Beitrage zur Physiologie der Holzgewachse. Fiinfstuck's 

 Beitrage z. wiss. Bot., Bd. I., 1897. 



t Pfefier, W. Pflanzenphysiologie, 2te Aufl., Bd. 11., p. 274, 1901. 



X Papers by Cannon, W. A. The Gall of the Monterey Pine. American 

 Naturalist, vol. 34, 1900; and Miss Mills in Entomological News, vol. 

 XI., 1900. 



