74 SCIENCE PROGRESS 



"autumn" wood is generally initiated about the middle of 

 August. Hartig found that in beeches, fifty to 150 years old, 

 the following fractions of the annual ring were produced : one- 

 third up to the middle of June, half up to the middle of July, 

 three-quarters up to the end of July and that the ring was com- 

 plete at the middle of August. Mischke obtained interesting 

 results by counting at intervals of several days the number of 

 tracheids constituting the growing annual ring of Pimts sylvestris 

 and Picea excelsa. Statistics in reference to the former are given 

 in the preceding table : in this specimen the production of new 

 tracheids commenced at the beginning of May and ended about 

 August 25. Three facts worthy of note were demonstrated in 

 connection with this slow-growing specimen of Pinus: (1) There 

 was a rise and fall in the rate at which the tracheids were 

 produced. (2) This general rise and fall was interrupted by 

 a complete stoppage in the production and maturation of the 

 tracheids between June 27 and July 24. (3) The outermost part 

 of the annual ring was ultimately constituted of several (three) 

 ranks of initial cells (that is, tracheids remaining in a juvenile 

 condition) destined to be converted in the following year into 

 " spring" tracheids. The same three features were noted in the 

 normally growing Picca excelsa : the cessation in the production 

 of tracheids lasted between June 27 and July 11 but in this 

 case some of the young tracheids already present thickened 

 their walls during the passive period. Mischke discovered, too, 

 that although no new tracheids were produced after August 25 

 the differentiation and lignification of the outer ones continued 

 during about a month longer. Mischke attributed the stoppage 

 in the middle of summer to the abnormally small rainfall in the 

 year of observation (1888) during April, May and June. But 

 this explanation cannot be accepted in the face of the evidence 

 supplied by D. Christison, who showed that the general rise and 

 fall in the rate of thickening of conifers was interrupted by a 

 marked fall in July in each of the three years of observation : 

 in two of these years the summer was unusually cold and wet, 

 whilst the other summer was exceptionally warm and dry. From 

 Dr. Christison's statistics I select as examples those dealing with 

 two species measured in 1888: as his figures are recorded for 

 unequal numbers of days, I have calculated from them the mean 

 diurnal increment during these unequal successive periods : the 

 results are given in inches in the succeeding table ; 



