486 TEXTBOOK OF BOTANY 



that are beneficial or those that are lethal. Only a few of the numerous 

 heritable differences that we see in plants are life and death differences. 



To one who tries to classify the plants of the world, the most funda- 

 mental heritable differences among them appear to be those that dif- 

 ferentiate the major groups, such as bacteria, fungi, algae, mosses, ferns, 

 and seed plants. Members of all these different groups of plants may be 

 found growing near each other in any natural forest, grassland and 

 desert, or even in water. Since they occur in most environments, the 

 great groups of plants became differentiated by heritable mutations that 

 have no survival value. Within each of these great groups, however, 

 species differ from each other by gene complements that have survival 

 value in some habitats. These points will be amplified in the next 

 chapter. 



Many mutations are of such a nature that the mutants cannot survive 

 in any environment. These are called lethal mutations. Because of them 

 some of the several fundamental processes, such as chlorophyll synthesis 

 and photosynthesis, fail to occur. If the lethal mutation is dominant, the 

 plant perishes. But if it is recessive, it may survive in the race indefi- 

 nitely, if there is sufficient cross-fertilization to keep it associated in 

 hybrids with the alternative dominant factor. Individuals that become 

 homozygous for the lethal mutation perish. 



Mutations that occur when plants are exposed to X-rays, extreme 

 temperatures, or other special conditions are reported to be of the same 

 kind as those that ordinarily occur in fairly uniform environments. It 

 appears, therefore, that the influence of environment is limited to 

 changes in rate of mutation and to the survival and distribution of the 

 new types of plants formed. 



Importance of scientific evidence. In view of the facts discussed in the 

 last four chapters it should be evident that the present generation is 

 aware of numerous botanical facts and principles that were unknown in 

 earlier times, and many of them were unknown or but vaguely glimpsed 

 before the beginning of the present century. These chapters could not 

 have been written today except for numerous recent discoveries in the 

 fields of physics, chemistry, and geology; and in the specialized fields of 

 botany: physiology, morphology, cytology, genetics, and ecology. The 

 present points of view are the results of more knowledge about the 

 fundamental nature of chemical stability and chemical change, a better 

 understanding of plant processes and development in relation to environ- 

 ment, and the integration of the facts of sexual reproduction, chromo- 



