436 TEXTBOOK OF BOTANY 



same plate is the red-flowered hibiscus plant, some branches of which 

 regularly bear white flowers. 



It thus became evident that three general types of variations should 

 be recognized in plants. There are, first, those variations that are the 

 direct results of the effect of environmental factors upon the conditions 

 and rates of processes within the plant, without anv changes in the 

 hereditary make-up of the plant or its progeny. We have referred to 

 many examples of this type of variation in previous chapters. Such 

 variations are non-heritable and are referred to as fluctuations. In con- 

 trast to fluctuations are the two types of heritable variations: the hybrid 

 variations resulting from cross-fertilization, and the mutations which are 

 changes in heredity not dependent upon cross-fertilization, though thev 

 may be increased by it. Their rate of occurrence is also influenced b\ 

 factors in the external environment, as we shall see later. The scientific 

 recognition of the causes and consequences of these three different kinds 

 of variations is largely a product of the research during the present 

 century. 



During all of this time, certain other botanists were interested in 

 studying the detailed structures of cells. The cytoplasm and nucleus 

 were clearly recognized and referred to as protoplasm about the 

 middle of the 19th century. During the latter half of the century the 

 chromosomes of the nucleus were seen, and their gross behavior 

 during cell division in both xegetative and reproductive cells was 

 recognized. During the present century it was discovered that nearlv 

 all of the known hereditar>^ factors are definitely associated with the 

 chromosomes. 



While the 19th century is noted for the discovery of many important 

 facts about plants, the final verification of many of those facts and of 

 their several dependent relations was not accomplished until the present 

 century. To those interested in plant and animal breeding, all these dis- 

 coveries are of prime importance because thev are the basis of a new sci- 

 ence known as genetics. To us in general botany they will help explain 

 many of our everyday observations; and as we proceed to amplif\' and 

 clarify them in the next few chapters it will become evident that they 

 constitute a basis of fact essential to an>' critical analysis of ideas about 

 evolution, inheritance of acquired characters, relative importance of 

 heredity and environment, adaptation, natural selection, and related 

 phenomena. 



