246 BOTANY: PRINCIPLES AND PROBLEMS 



ever, there began to appear within it the same tendency which 

 manifests itself in the evolution of a human society, the "division 

 of labor". Instead of the primitive condition in which all the 

 individual cells carry on all the functions, certain cells became 

 specialists, some of them gradually assuming the performance of 

 one function and some of another, and acquiring through this 

 specialization a more or less conspicuous modification in struc- 

 ture. The first activity of plants to be thus locahzed was prob- 

 ably reproduction. Instead of a condition where every cell 

 divided and gave rise to new individuals, certain ones were set 

 apart to produce specialized reproductive cells or spores, provided 

 with means of locomotion or other facilities which made them 

 particularly well adapted to establish a multitude of new and 

 widely scattered plants. This process of differentiation has stead- 

 ily progressed during the evolution of the plant kingdom and has 

 resulted in the marvelously complex individuals which we have 

 studied among the seed plants. Here the various functions 

 have organs devoted to their performance and in these the cells, 

 far from being uniform, are grouped in definite and highly 

 specialized tissues, each of which plays its particular part in the 

 life of the whole. Differentiation has made possible the existence 

 of the higher plants, and is one aspect of that phenomenon of 

 organization or regulation to which we have so often called 

 attention. 



3. Sexual Reproduction. — Another important step in the 

 history of the plant kingdom involved the method by which 

 reproduction took place. In the earliest plants, this process was 

 accomplished merely by a division of the cell into two. In 

 forms a little more advanced, special cells became differentiated, 

 each of which was able to produce a new plant. Following this 

 stage, the type of reproduction which we know as sexual probably 

 made its appearance. The essential feature of this method is 

 the fusion of two cells into one and the subsequent development 

 therefrom of a new individual (Fig. 137). The cells which thus 

 unite are called sexual cells or gametes, and the product of their 

 union, the zygote. In early plants, the gametes were probably 

 nothing more than the ordinary non-sexual reproductive cells 

 which had assumed this additional function; and in some of the 

 algae today we find cells of this sort, which may reproduce the 

 plant either sexually or asexually. Soon the gametes became 

 clearly distinct, however, and asexual reproduction was often 



