178 GROWTH HABITS OF PLANTS 



methods of increase and the character and length of life of the 

 vegetative shoots, consists of four large groups; namely, annuals, 

 biennials, herbaceous perennials, and woody perennials. 



Annual plants pass the unfavorable season in the seed or spore 

 form, and the vegetative shoots live during only one growing season, 

 the entire length of life of the plant being about one year. Biennials 

 pass one unfavorable season as seeds or spores and the next as a 

 vegetative propagating organ of some form. There is no accumula- 

 tion of aerial shoots and the plant lives during two or parts of two 

 years. Herbaceous perennials pass each unfavorable season both in 

 the seed or spore form and in the form of vegetative propagating 

 parts. There is no accumulation of aerial shoots but the plant lives 

 several or many years. Woody perennials pass each unfavorable 

 season in the form of seeds or spores or as plants with aerial shoots. 



For more accurate description of vegetation types the last two 

 groups, herbaceous perennials and woody perennials, may be sub- 

 divided as shown in the following list. 



113. The Evolution of Growth-forms.— Our knowledge is not yet 

 sufficient to enable us to arrange all of the growth-forms of plants in 

 an evolutionary sequence which we can be sure is correct, and per- 

 haps it never will be. But a beginning has been made by a number 

 of workers, and it is now believed that among the angiosperms the 

 woody type is more ancient than the herbaceous. Probably the 

 most primitive type of tree is the sparingly branched, rain-forest 

 type with large, undivided leaves. During the course of evolution 

 more xeric forms have been produced and there has been a tendency 

 toward reduction in size of plant and of leaf, an increase in the 

 amount of branching of the plant and of the veins in the leaves, the 

 production of compound leaves and of deciduous leaves, and the 

 development of thorns and of succulence. Climbing plants, epi- 



