2 Wisconsin Academy of Sciences, Arts, and Letters. 



the results obtained from a histological study of the early stages 

 of crown-rot. 



The purpose of this paper, then, is to summarize in some de- 

 tail most of the important hypotheses and investigations dealing 

 with the matter included in the title, to compare them with one 

 another and to bring out their relation to the writer's observa- 

 tions. Thus collecting the widely scattered ideas and summariz- 

 ing the records of research along this line, it is hoped will stim- 

 ulate a wider interest in the causes of periodic growth in trees 

 and encourage and lead to their reconsideration from a more 

 modern or quantitative standpoint. In the main the aim is to 

 restate the questions raised by the investigators, although some- 

 times in a modified form. A restudy of the structural and 

 tension changes accompanying periodic growth may also lead to 

 an investigation of the enzymes active during radial growth and 

 to the effect which adverse changes of environment have 

 upon them while in an active condition. In any case studies of 

 this type will throw more light on the relation of a varying en- 

 vironment to vegetative and reproductive processes in woody 

 plants and thereby increase the knowledge necessary for a com- 

 prehensive investigation of their diseases. Most of the diseases 

 of trees which are of much economic importance and of most 

 scientific interest begin in the bark, and their origin seems to 

 have a definite relation to such radial growth and consequent 

 bark tensions, the normal adjustment of which is interfered with 

 by subsequent changes in the environment. Studies of that 

 kind will also help to clarify and perhaps correct some misappre- 

 hension that may exist regarding the relation of mycology and 

 physiology to plant pathology. 



Seasonal Periodicity op Growth. 



It is generally held that seasonal periodicity or the alterna- 

 tion of one or more growing and resting periods during the year 

 is a more or less unalterable inheritance of perennial plants of 

 temperate zones, but KLebs starting with his extensive investi- 

 gations on the artificial control of periodicity in algae and fungi, 2 

 has reached a very different conclusion regarding the periodic 



' Klebs, G. Willkiirliche Entwickelungsanderungen be Pflanzen. pp. 166. 

 Jena, 1903. 



