IV. Correlations in Development. 263 
root apices and a consequent limitation of their rate of growth. If the stem 
for any reason stopped growing, we should expect the roots to grow at an 
increased rate. 
In Nos. 7 and 8 of this series the shoot was removed, and a marked 
increase in the rate of growth for the next four days was the result of this 
operation. The rate of root growth then fell again to a low rate, and about 
the fifth or sixth day new shoots were visible, the falling off in root growth 
apparently being due to their development. 
These assumptions were examined further by growing peas ( Pisum 
sativum) and determining the average weight of stems and of roots at fixed 
intervals. In order to avoid complications due to photosynthetic effects on 
dry weight the peas were grown in the dark. This does not appear to alter 
materially either the rate or the total amount of root growth. Attempts 
were also made to eliminate some of the extreme variability observed in 
some earlier experiments.. Two main types of variation were usually 
observed : (1) variability in germination, presumably due to variations in the 
thickness and permeability of the seed-coat; (2) variability in subsequent 
development. To get a moderately uniform and fairly representative set of 
growing peas, about three thousand peas were spread out and soaked in 
sufficient Shive’s optimum three-salt solution to just cover them. On ger¬ 
mination about three hundred of these were selected, having roots of average 
and similar length (1 *5-2*5 cm.), the extreme variants being thus eliminated. 
The selected plants were then grown with their roots dipping through 
a perforated plate into the Shive’s solution. The cotyledons rested upon 
filter-paper and the seeds were kept in incubators at fairly constant tempera¬ 
ture and humidity. The nutrient solution was changed every three days. 
In Series III no further elimination of variations was made, but in 
Series IV to VI no determinations were made until the appearance of 
secondary roots—and then the material was graded according to the length 
of the shoots, equal numbers of plants with very long or very short shoots 
being discarded. In this way only about 100 to 150 plants were left, the 
extreme variants having been eliminated at two stages in the growth-cycle 
on two criteria, (1) root length, (2) stem length. 
The samples for determination of the dry weight were taken in lots 01 
ten. The attachment to the cotyledon was treated as part of the root— 
since it increases in weight in proportion to the root—and after cutting the 
plant away from the cotyledons at the junction of the attachments and 
cotyledons, the stem was also cut off just above the attachment. The 
roots, stems, and cotyledons were then dried for six hours at ioo° C. and 
parts of each plant weighed separately. The average dry weight and 
probable error of the set of ten plants were then determined. 
A modification in the method of sampling was introduced in the case 
of Series III, IV, and VI. In these series, equal numbers of the extreme 
