876 



SCIENCE 



[N. S. Vol. XXV. No. 648 



A NEW EXPLANATION OF THE TOLERANCE AND 

 INTOLERANCE OP TREES 



Before a recent meeting of the Society of 

 American Foresters Mr. Kaphael Zon read a 

 paper presenting the new explanation of the 

 tolerance and intolerance of trees. 



The theory of tolerance as formulated by 

 Pfeil and Gustav Heyer and the classification 

 of trees into light-needing and shade-enduring 

 is the foundation upon which the forester 

 bases all his practical work in silviculture. 

 Many biological, ecological and silvicultural 

 facts have, however, been accumulating which 

 tend to show that the increased growth of trees 

 after thinning or the possibility of securing 

 reproduction of certain species only in full 

 light or after heavy thinnings, are due chiefly 

 to temperature and moisture condition, and, 

 not, as has been supposed, to light require- 

 ments. It has been found, for instance, that 

 the same structural differences which occur 

 between trees growing in the open and trees 

 growing in the shade, such as the reduction of 

 leaf surface, the diminution of intercellular 

 spaces, the lengthening of the palisade cells, 

 etc., occur also between trees growing in dry 

 or in moist situation, or in a dry or humid 

 atmosphere. It has also been observed that 

 trees within the same climatic region are more 

 tolerant of shade when grown in fresh or 

 moist soils, than when grovm on dry or poor 

 soils. 



Fricke, a Geitman silviculturist, has proved 

 recently by a number of very interesting and 

 convincing experiments that the failure of 

 Scotch pine to grow under the shade of mother 

 trees was not due to lack of light, but to de- 

 ficiency of moisture, which is a result of com- 

 petition with the roots of the larger trees. 



In a stand of pine about one hundred years 

 old, with a crown density of 0.7, growing on 

 poor sandy soil, where the light requirements 

 of pine are greatest, a number of isolated 

 groups of suppressed young pines were 

 located. The young pines were ten years old 

 and about a foot and a half high. These 

 groups were surrounded by ditches so that the 

 roots of the neighboring large trees were cut 

 through to a depth of ten inches. The little 



trees within the ditched areas immediately 

 responded. The needles had double the length 

 of the preceding summer, the terminal shoots 

 became longer and the growth generally 

 thrifty and has continued so, while the young 

 growth not surrounded by ditches retained the 

 same suppressed character. Subsequently he 

 determined by analyses that the soil moisture 

 content in the areas surrounded by ditches was 

 from 30 to 40 per cent, higher than that on 

 contiguous areas not ditched and penetrated 

 by the roots of living trees. 



It must, therefore, be conceded that the 

 moisture conditions play an extremely im- 

 portant part in determining the behavior of 

 trees growing in shade and in light, and that 

 the role which light plays is by no means an 

 all-important one. The theory of the toler- 

 ance and intolerance of trees as hitherto 

 understood is not tenable and must be thor- 

 oughly revised. 



A LABORATORY MANUAL 



Some time ago Bergen and Davic'a book 

 entitled ' Principles of Botany ' was noticed in 

 these columns (Science, January 25, 1907). 

 We have now a companion or supplementary 

 volume in the ' Laboratory and Field Manual 

 of Botany ' (Ginn) by the same authors, in- 

 tended for the use of pupils in high schools, 

 and perhaps in the smaller colleges. It in- 

 cludes ten or eleven chapters on such topics 

 as laboratory methods and equipment (fuU of 

 excellent suggestions) ; structure and physi- 

 ology of seed plants (accompanied with forty- 

 two suggestive experiments) ; type studies, pre- 

 ceded by the study of the plant cell (beginning 

 with simple forms and passing regularly to 

 higher and higher tyi>es) ; ecology (accoBi- 

 panied with many studies) ; botanical micro- 

 technique (full of excellent advice) ; culture 

 methods (containing much of great praetical 

 value) ; material, apparatus, and supplies (a 

 most helpful chapter for the teacher) ; bibli- 

 ography (carefully classified) ; glossary, etc. 

 The book is so evidently the result of years 

 of experience by the authors in the supervision 

 of botanical laboratory work that we are not 

 surprised at its air of practicality and work- 



