Page:Popular Science Monthly Volume 84.djvu/140

136 differential mortality with respect to seed weight, I have combined in Diagrams 2-3 the data from the field trials already published with those from a series of experiments on germination in sand. The lumping of the two sets of experiments which differ in some slight but apparently significant details to give sufficient series to make smooth graphs is justified by the fact that individually they lead to essentially the same conclusions and that the data and minute comparisons are to be presented in full detail eventually.

Diagram 2 shows the relative differences in type (mean seed weight) between the total samples of seeds weighed and those which produced plants.

Here the heavy vertical bar represents zero difference between the average weight of the total population of seeds and those which actually produce plants. The broken lines and circles to the right show on the scale at the bottom, where each unit represents one tenth per cent, the number and amount of the positive differences, that is to say of those in which the seeds which survived are heavier. The heavy lines and solid dots to the left of the zero bar indicate the number of experiments giving negative differences—i. e., in which the mean weight of the series of seeds which produced plants was less than that of the general population—and the amount of the difference in relative weight.

Judging the areas of light and dark shading, by the eye alone, one would conclude that the surviving seeds are slightly heavier than the population from which they were drawn. But the deviation from the equality of division which would be expected if there were no relationship between the weight of the seed and its capacity for survival is only 4 ± 2.98 cases, and little significance can be attached to it. For the