74 PLANT LIFE OK THE FARM. 



The limits of variation no one can tell ; sometimes they 

 seem very narrow ; at other times we know them to be 

 very wide. Within short periods of time the amount of 

 variations may be inappreciable. Within geological 

 periods the variation in the course of development is 

 sometimes so enormous that, were there not evidence of 

 the fact, it would be difficult to connect the plants and 

 animals that have gone before with those which now 

 exist. Living plants, then, are influenced in the course 

 of their development by two somewhat antagonistic prin- 

 ciples the hereditary principle which, on the whole, 

 tends to keep plants as they are, and the tendency to 

 vary, which is the source of that variation in character 

 which enables plants and animals gradually to become 

 adapted to altered circumstances. This is beneficial to 

 the cultivator, by affording him an opportunity of select- 

 ing the varieties best suited for his purpose. It is by 

 exercising selection of this kind that Mr. Hallett suc- 

 ceeded in raising his "Pedigree Wheat." He simply 

 selected for sowing the best grains from the largest and 

 best ears, and repeated the process year after year, just 

 as the gardener has done for countless generations in the 

 case of fruits and .seeds, as the cattle-breeder does with 

 Shorthorns or other pedigree animals. These processes 

 the farmer might with great advantage practice to a 

 much greater extent than he usually does, and thus se- 

 cure hardy productive varieties best suited to his particu- 

 lar conditions and best likely to fulfil his requirements. 



Formation of the Embryo, The two processes of 

 growth and development may also be illustrated by recall- 

 ing what takes place in the germination of a seed. A 

 ripe seed contains within its coat or husk an embryo 

 plant. Very often that embryo plant is invested, as in 

 the case of the grain of wheat, with a whitish, floury 

 substance, known as the "perisperm." All grass seeds 



