Maech 3, 1916] 



SCIENCE 



323 



families than do woody plants, but tlieir number 

 of species per family is greater. A study of the 

 rapidity of evolution in these two types throws 

 light on the antiquity of the angiosperms. 

 Experimental Evolutionary Investigations with 



the Genus Drimys: E. C. Jeffrey and K. D. 



Cole. 



The genus Drimys of the Magnoliaceae has long 

 excited interest as a dicotyledon entirely without 

 the vessels, which are such characteristic features 

 of wood -structure in the angiosperms as a group. 

 The value of experimental work in connection with 

 evolutionary problems has been particularly empha- 

 sized in recent years and there can be no doubt 

 that experimental studies on the part of those 

 who are sufficiently acquainted with the history 

 and morphology of plants to interpret the mean- 

 ing of the structures experimentally produced, is 

 of great value. Experimental investigation has in 

 the past few years thrown great light upon the 

 evolutionary history of the conifers and has be- 

 gun to be applied to the elucidation of the course 

 of evolution to the dicotyledons. The present 

 communication is for the purpose of calling atten- 

 tion to the highly interesting fact that structures 

 resembling vessels can be recalled in BrimyS as a 

 result of experimental procedure. It has not yet 

 been found possible to bring about the return of 

 such structures in the stem^ but vessels are readily 

 recalled in that most conservative of all plant or- 

 gans, the root. The experimentally recalled vas- 

 cular structures resemble those found in the Mag- 

 noliaceae as a whole. The consequence of this 

 demonstration is of considerable evolutionary im- 

 portance on account of the primitive position often 

 accorded to the Ranales and in particular to the 

 Magnoliaceae. It seems clear that Drimys can no 

 longer furnish an argument in favor of this view 

 since the simplicity of wood structure, resembling 

 that of the Conifers, is not primitive but clearly 

 the result of reduction. 



Is the Vesselless Secondary Xylem of Certain 

 Angiosperms a detention of Primitive Gymno- 

 sperm Structure? W. P. Thompson and I. W. 

 Bailey. 



Vessels are entirely absent in the xylem of Tetra- 

 centron, Trochodendron and Drimys. Vestiges of 

 vessels do not occur in the root, seedling, young 

 stem, petiole, traumatic tissue, and other regions 

 that have been considered to be retentive of an- 

 cestral characters. The form, structure and ar- 

 rangement of the tracheids of the xylem closely 



resemble those of gymnosperms, and there seems 

 to be no valid reason for not considering the three 

 genera primitive as far as their xylem structure is 

 concerned. The wood-parenchyma is "diffuse" 

 in Tetracentron, Trochodendron and Drimys win- 

 teri. In D. colorata and D. axillaris it shows 

 transitions from diffuse to banded and terminal. 

 The distribution of parenchyma in these three 

 genera makes it seem very improbable that the 

 "terminal parenchyma" of the Magnoliaceae orig- 

 inated through reduction from the " vasicentric " 

 condition. There appears to be no reliable evi- 

 dence to indicate that the Magnoliaceae and allied 

 families are forms that have become highly spe- 

 cialized through ' ' reduction ' ' from advanced 

 types of Angiosperms. 

 Some Observations upon the Secondary Xylems of 



Gymnosperms and Angiosperms : W. W. Tupper 



and I. W. Bailey. 



From measurements of tracheid-lengths and 

 wood-fiber-lengths taken from the secondary xylems 

 of a large number of gymnospermous and angio- 

 spermous woods, respectively, the authors have con- 

 firmed and supplemented the work of Sanio, Be 

 Bary, Becord and others, which show a very great 

 difference between the lengths of gymnospermous 

 tracheids, on the one hand, and the tracheids and 

 wood-fibers of the dicotyledons, on the other. 



The former elements average more than twice 

 as long as do those of the dicotyledons of about 

 the same age, with the very striking exception of 

 the vesselless angiosperms, Tetracentron, Trocho- 

 dendron and Drimys, which seem to have the typ- 

 ical gymnospermous length of wood elements. 



Climaxes and Climates of Western North America: 



Frederick E. Clements. 



Each climax vegetation is regarded as a de- 

 velopmental unit, and hence is designated as a 

 formation. The vegetation of the continent is con- 

 sequently made up of a number of climax forma- 

 tions, each with a development and structure more 

 or less peculiar to itself. As a result, it becomes 

 desirable to distinguish two kinds of units, de- 

 velopmental and climax, the one typical of suc- 

 cession, the other of the final adult condition of 

 vegetation during a particular climatic period. 

 Each climax is coextensive with its climate, and 

 is in fact the indicator of the latter. Climaxes, 

 moreover, exhibit a phylogenetie sequence due to 

 differentiation and shifting in the face of a cli- 

 matic crisis, such as glaciation. Such a shifting is 

 recorded in the zones, such as are characteristic of 



