7 6 



THE BOOK OF NATURE STUDY 



Wheat is grown successfully when the average July temperature 

 is at least 56 F. and the rainfall below thirty-three inches. More 

 wheat is grown in England than in Scotland ; more in the south 

 than in the north. The line of the northern limit of wheat passes 

 through Britain. There is also a well-marked limit in altitude, 

 but this varies with the slope of the hills. Mr. R. Smith gives 

 five hundred feet as the limit on the northern slopes of the Pent- 



lands near Edinburgh, and seven hundred 

 on the south-eastern slopes; in Yorkshire, 

 wheat grows well at an altitude of six or 

 seven hundred feet. 



WHEAT, AN " INDICATOR "-PLANT. In 

 ecological observations Wheat has been 

 taken as an indicator-plant. It is found, 

 for instance, that certain trees and weeds 

 do not ascend higher than the Wheat line. 

 It has been observed that the lowland oak 

 woods within the Wheat zone have a 

 richer vegetation than oak woods above 

 the Wheat limit. The same holds good 

 with the weeds of arable land. Of sixty- 

 three out of one hundred weeds found in 

 Yorkshire to be common to arable land, 

 only forty-two occur above the Wheat 

 line. The Common Poppy, the Wild 

 Radish, Field Pansy (Viola arvensis), the 

 Corn Cockle, Sow-Thistle, Spurge, Shep- 

 herd's Needle are some of the most pro- 

 minent in the wheat zone. Certain aquatic 

 plants, such as the Bur-reed (Typha lati- 



folia) and the Common Reed (Phragmites communis) are distributed 

 over the same area ; whilst others belong entirely to marshy 

 ground, where wheat cannot be grown. The Corn Cockle (Lychnis 

 Githago) is a remarkably handsome plant, with a peculiar calyx, 

 which has long, green linear lobes projecting beyond the petals. 

 The fruits of the Wild Radish and the Pansy are worth noticing : 

 that of the Radish is jointed by transverse partitions into as many 



FIG.SI. Corn Cockle (ydfc 

 Githago}. 



