292 The Plant World. 



germination and seedling mortality on a limited area. My work 

 on this area has been confined to six common types, four of 

 cacti and two of sclerophyllous trees. In addition to deter- 

 mining their germination behavior I have secured continuous 

 records of the atmospheric evaporation rate and the moisture 

 of the soil at depths of 3, 15 and 30 cm. 



In a previous papei "^^ I have reported on statistical work 

 with the Giant Cactus, which went to show that it is not maintain- 

 ing itself at its former rate of establishment. I have since en- 

 deavored to get similar statistics for the Palo Verde {Parkin- 

 sonia micro phylia), which is a Leguminous tree, with chloro- 

 phyll-bearing bark and very small leaves, which are borne onlv 

 in the rainy seasons of late winter and midsummer. In the 

 stems of Palo Verde very nairow rings of giowth are visible. 

 By polishing and varnishing sections of trunks it is possible to 

 count these rings by the aid of a hand glass. By counting the 

 rings along several radii, and by recounting each section after 

 I had recorded and forgotten how many rings it had, I was able 

 to determine the number with no errors in my extreme differ- 

 ences of count of more than 10%. The fact that Palo Verde has 

 two periods of foliation in the year made it seem probable that it 

 might have two periods of growth in trunk diameter, although 

 I had noted that its shoots made no growth of elongation in the 

 late winter rainy season. In order to determine whether the 

 rings of the trunk are annual or semi-annual I made a careful 

 measurement of some thirty stem diameters on six individuals 

 in two localities. The places at which the measurements were 

 first made were marked by touches of paint, which proved to do 

 the stems no injury. The measurements were made with mi- 

 crometer calipers, reading to .1 mm. The result was that no 

 growth was detected during the late winter period of foliation, 

 showing the rings to be annual. In order to secure a curve of 

 growth rate I cut 22 trees of difl'erent diameters and counted 

 their rings. I then secured trunk diameters of a j)opulation of 

 146 treees on the north slope of Tumamoc Hill, over the same 

 area on which I made one of my determinations of establish- 

 ment rate in the Giant Cactus. The trees were measured with 



*Shreve, Forrest — The Rate of EstabJishinent of the Giant Cactus, Plant World, 13, 235- 

 240. 1910. 



