90 AGRONOMY 



and remain dormant during the summer, but in autumn begin 

 to grow again, making more or less root growth all winter. 

 The white, or Madonna, lily rests for a time in summer and 

 makes new growth in autumn. Possibly half the species of 

 crocus produce their flowers in autumn, and the witch-hazel is 

 a well-known shrub with the same habit. Greenhouse plants, 

 coming as they do from a region in which the season of rest 

 is the dry season, are benefited by withholding water after 

 the season for growth is over. Calla lilies and other bulbous 

 plants are often allowed to become entirely dry after flowering. 

 Our own plants seem to be adjusted to a season of cold for 

 the rest period, though in many cases an exposure to dryness 

 is as effective. In cold climates hardiness is often a matter 

 of complete dormancy. 



Genera, species, and varieties. Although each kind of plant 

 has adopted the form most suited to its position in life, and 

 in consequence has become different from all others, this has 

 not resulted in a multitude of disconnected forms. Strong 

 lines of resemblance run through the different groups, and, 

 interwoven through the vegetable kingdom, bind it into one 

 related whole. As a general thing, the more decided the re- 

 semblance between different forms, the closer the relationship. 

 There are certain types of leaf and flower on which nature 

 has rung a thousand changes, producing plant after plant 

 essentially alike though ever varied. The casual observer 

 do.es not fail to note these differences, and usually recognizes 

 groups like the violets, lilies, asters, grasses, and legumes at 

 sight, though he may fail when it comes to the lesser distinc- 

 tions that separate species from species. The botanist, how- 

 ever, finds it convenient to carefully delimit these smaller 

 divisions. To the unit of his classification he gives the name 

 of (species and defines it as a group of like individuals. All 

 the plants of white clover or of field corn form a species. 

 Crenera (singular, yenuz) are groups of related species. Ked 



