Building off his previous previous work, Woodbridge again depicts isothermal conditions. Here, he updates the way the planting information is displayed (in boxes, with horizontal text). Overall, Woodbridge notes that the figures on the chart indicate mean annual temperature of the places depicted. The dotted lines crossing the chart point out the places which have equal degrees of heat. Theres form the boundaries of the Regions distinguished by color.
What is also interesting about this map is that it appears to also be a planting calendar, suggesting what crops are best suited for each Region. These are depicted by the unbroken vertical lines reaching from each of the dotted Regional lines.