June 2, 1922] 



SCIENCE 



583 



of the plant. Inasmuch as publication of the 

 details of these investigations has been consid- 

 erably delayed it seems desirable at this time to 

 briefly indicate the principal conclusions 

 reached. The duration of the daily illumina- 

 tion period not only influences the quantity of 

 photosj'nthetic material formed but also may 

 determine the use which the plant can make of 

 this material. In general, there is aij optimal 

 light period for maximum upward or apogeo- 

 tropic elongation of the stem which for some 

 species corresponds to the long summer days of 

 higher latitudes, while for other species the 

 intermediate length of day of spring and fall 

 (or the equatorial day length) is optimal. 

 Changes in the light period to sub-optimum 

 conditions for stem-elongation, resulting from 

 appropriate increase or decrease in length of 

 day, as the case may be, may initiate a series 

 of characteristic responses which are definitely 

 associated with periodicity in plant behavior. 

 Eeference has already been made to flowering 

 and fruiting. There seems to be an optimal 

 light period for sexual reproduction which 

 tends to direct the energies of the plant more 

 or less quantitatively toward flowering and 

 fruiting. Again, departure in day length from 

 the optimal for increase in stature causes loss 

 of dominance of the apical bud, thus promoting 

 various types of branching. Leaf-fall and en- 

 trance Tipon the rest period, also, result from 

 exposure to a certain length of day which is 

 unfavorable for stem-growth. It has been 

 found that there may be an intermediate length 

 of day especially favorable to dormancy or 

 death while under both longer and shorter days 

 activity of the p)lant may continue. Further 

 changes of the light period by a sufficient incre- 

 ment or decrement away from the optimal for 

 increase in stature and beyond the optimal for 

 sexual reproduction tend to induce intense 

 tuberization, a feature marking the final stages 

 in reduction of stem-elongation. Formation of 

 bulbs is induced by excessively long days while 

 formation of tubers commonly results from ex- 

 cessively short days. This deposition of carbo- 

 hydrate in relatively condensed or dehydrated 

 forms as a result of an unfavorable light period 

 indicates marked loss of power to utilize the 

 products of photosynthesis in elongating the 

 stem or in developing flower and fniit, a con- 



dition well exemplified in the stemless or leaf- 

 rosette form of foliage development. The 

 opposite change toward the optimal day length 

 for stem-elongation may rescue typical annual 

 plants from impending death and effect more 

 or less complete rejuvenescence. The evidence 

 indicates that the degree of hydration of the 

 living cell content is brought under delicate 

 control by the ratio of the number of, hours of 

 sunlight to the number of hours of darkness 

 in the 24-hour period. Well defined correlation 

 has been established between the hydrogen-ion 

 concentration of the cell sap and the observed 

 responses of the plant to change in the length 

 of the day. Thus, change from the purely 

 vegetative to the flowering and fruiting stage 

 may involve marked change in hydrogen-ion 

 concentration in the apical bud and even a 

 reversal of acidity relations between the apex 

 and the base of the stem. Correlation also has 

 been found between the content of "available" 

 carbohydrate (the simpler sugars) and the 

 responses of the plant to differences in length 

 of day. Causal relationships, however, have 

 not been definitely established. It seems prob- 

 able that the annual cycle of length of day, 

 affording as it does a consistently rhythmic 

 feature of the external environment, is a dom- 

 inant causal factor in phenomena of plant 

 periodicity, subject, of course, to the modifying 

 influences of temperature and other environ- 

 mental factors. 



W. W. Garner 

 H. A. Allard 

 Bureau op Plant Industet, 

 u. s. t)epartment op agriculture 



FISH PARASITISM IN ITS RELATION 



TO BIOLOGICAL PROBLEMS OF 



THE NORTHWEST! 



■ In this great Northwest of ours fish afford a 

 natural resource of importance to the welfare 

 of a good many citizens. Not only do the 

 commercial interests utilize fish for market 

 purposes, but the sportsmen derive infinite 



1 One of the papers in a Symposium on ' ' Biol- 

 ogy in Its Relation to the Development of the 

 Northwest," presented at the meetings of the 

 Western Society of Naturalists at Corvallis. 



