86 CARNEGIE INSTITUTION OF WASHINGTON. 



Varieties of these two plants cultivated in the region of the Desert 

 Laborator}^ were selected, and grains were germinated in an unheated 

 glass-house. The bases of the plantlets were fixed in place by layers of 

 plaster poured on the surface of the soil. The tips of leaves which 

 had emerged to a length of 10 to 15 mm, were brought into the field 

 of a horizontal microscope and the variation in length measured at 

 half -hour intervals so far as it was possible to do so. The leaves were 

 maintained in a vertical position by a requisite number of horizontal 

 glass rods with a minimum of shading effect. The increments meas- 

 ured are of course inclusive of the elongation of the base of the leaf and 

 of the internode from which it arises, as well as of any residual action 

 of internodes. 



Retardation of growth of Zea and Triticum occurs at more than one 

 place in the temperature scale and at different times of the day. An 

 uneven rate of elongation w^as particularly noticeable in Triticum, 

 although displayed by Zea as well. It was thought that the UTegu- 

 larity might be due to a sagging of the leaf-blade which would cause 

 its tip to move with a varying rate across the field. Similar leaves 

 attached to the bearing arm of an auxograph under a stretching 

 tension traced an undulating line indicative of similar irregularities. 

 Cessation of growth, especially in some of the instances in Zea, may be 

 reasonably attributed to a direct temperature effect, especially in the 

 cases in which the thermometer stood at 30° C. to 35° C. for extended 

 periods. In the greater number of instances, particularly in Triticum, 

 no such explanation could be deemed adequate, and the matter is re- 

 ferred to varying imbibition capacilA^ coincident with variations of 

 acidity due to respiration modified by various agencies. 



The highest rate that was maintained for some time hj Zea was 

 found to He between 27° C. and 30° C. It is not possible to fix upon 

 any limits of temperature within which growth might be continuous 

 in this plant. It is obvious that "secondary" maxima might readily 

 be derived from data of this character. 



No retardations occurred except after 11 a.m. in either Zea or Trit- 

 icum, and while Zea showed an acceleration late in the day after retard- 

 ation at high temperatures, Triticum did not. The retardations in 

 question are relatively least in the earlier stages of development, when 

 the joints are not more than one-fourth or one-fifth adult size and 

 have the effect of a flattening of the curve that is of slowing-down 

 growth. The action becomes more pronounced until a stage is reached 

 when more and more of the elongation of the forenoon is retracted in 

 the afternoon. The tonic range of the two plants is of course not iden- 

 tical. Wheat grows at a lower range than corn and probably reaches 

 its upper limit near the figures given. 



There are but three allowable causes in the present state of our 

 knowledge, to which might be attributed the slackening or inhibition 



