782 EXPERIMENT STATION RECORD. 



in the trees is usually represented by a zone of 6 or 7 greatly reduced 

 rings. If each ring represents a year's growth the zone should begin 

 with the seventy-seventh from the bark. In some cases this was found 

 to be true, but more frequently there was a discrepancy of 3 or 4 rings. 

 Counting from the inside of the zone of reduced rings to the bark a 

 similar discrepancy is noted, showing that the number of rings can not 

 be taken as a ])erfect index of the age of a tree, but shows the number 

 of growth periods, which may be influenced by cold or drought, in which 

 case several rings may be deposited during a single year, or in (trdinary 

 seasons an occasional extra ring may be deposited. 



The paths of gaseous exchange between the aerial leaves and 

 the atmosphere, F. F. Blackman {Proc. Hoy. ISoc.y'j? {1895), No. 343, pp. 

 105-168). — On account of the smallness of the quantities of gas involved, 

 the author states that practically no attempt has been made hitherto to 

 determine by direct estimation the question of the path by which carbonic 

 acid passes out of the leaf in resi>iration and into it in assimilation — 

 whether this takes place by the stomatal o[)enings or through the con- 

 tinuous surface of the cuticle. The existing experimental evidence is 

 all of an indirect nature, and rather tends to support the view that the 

 exchange is a outicular phenomenon. Barthelemy in 1SG8 put forth the 

 view that the cuticle Avas especially adapted for transmitting carbon 

 dioxid from the external air to the assimilating cells beneath. This 

 view he supported by exjjeriments on the artificial osmosis of gasses 

 through leaves. About the same time Boussingault perforjned ex]>eri- 

 ments that seemed to show^ definitely that in assimilation the carbon 

 dioxid taken up by the leaf entered it through the upper surface devoid 

 of stomata rather than through the more distant stomatal openings. 

 In support of the view that stomata form the paths of gaseous exchange 

 we have the conclusion arrived at by Mangin in 1888, from diffusion 

 experiments on isolated cuticle, that this diffusion is insufficient to 

 account for the whole gaseous exchange of the leaf. By the aid of an 

 apparatus designed by the autiior he has been able to estimate suc- 

 cessfully the amount of carbon dioxid given out and taken in by the 

 two surfaces of the same leaf under the same conditions. For this 

 pur])ose shallow capsules 10 sq. cm. in area, consisting of a glass plate 

 with metal rim, through which tubes for the circulation of the air current 

 pass, are employed. Two of them are affixed to a leaf on opposite sides 

 of the same area in air-tight union by means of soft wax. Through 

 these two continuous currents of air cau be kept flowing over the two 

 surfaces and the carbon dioxid produced or taken in during a given 

 time by each of them be determined. JSTumerous experiments on the 

 respiration of a variety of leaves with the stomata all on one side or 

 variously distributed on the two sides agree in showing that the 

 stomata are the site of the exhalation of this gas. When no stomata 

 are present on the upper surface ot the leaf then practically no carbon 

 dioxid is exhaled from that surface, while more than thirty times as 



