THE PLANT BODY 



root is a basic part of the axis; adventitious roots, with simple, crude 

 attachment to the mother axis, are secondary organs. 



Variety in growth habit ranges from tree and shrub to herb and 

 vine, woody and herbaceous. The diverse habital forms represent many 

 types and stages of specialization along parallel and convergent lines. 

 Distinction between trees and shrubs is based chiefly on differences in 

 height, woodiness, and permanence, but all habital types merge with 

 others. Some herbs are taller than some trees. Many herbs, even annuals, 

 have strong, woody stems, and some tropical trees have weak, "fleshy" 

 trunks. Vines are both woody and herbaceous, with major and minor 

 structural adaptations to the climbing habit. The tree habit is ancient 

 among vascular plants. Trees of good size were present in the Devonian 

 period and have been present in major taxa since that time. Because of 

 their large size, trees are prominent among angiosperms: it has been 

 estimated that there are 20,000 to 25,000 living species. Great height — 

 over 100 feet — is attained in many families; 200 feet and over in a few. 

 The Australian gums are, without doubt, the tallest angiosperms; 

 Eucalyptus regnans reaches a maximum height of 326 feet. (Taller 

 individuals reported are probably mythical.**) Herbs greatly outnumber 

 woody plants in genera and species. Herbs are prominent in some areas 

 because of the vast numbers of individuals. Their early evolutionary 

 history is unknown; their softer structure is not so likely to be preserved 

 in the fossil record as that of woody plants. 



THE PLANT BODY 



The gross structure of the plant body has been variously interpreted, 

 morphologically. In early days, it was considered to be made up of 

 several "fundamental parts" or organs — root, stem, leaf, floral organs, 

 ovules, trichomes; in recent years, the number of these basic organs has 

 been reduced to three — root, stem, and leaf; in present usage, the stem 

 and its appendages are commonly considered as a unit, the shoot. 



Recognition of the shoot as a fundamental part has been a gradual 

 process. The concept that stem and leaf together form an entity was 

 probably first proposed at the beginning of the twentieth century. In 

 succeeding years, three interpretations of the make-up of the shoot 

 were proposed: (1) that the stem is the fundamental part, and the 

 leaves appendages; (2) that the leaf is the basic part, and the stem 

 consists wholly or in large part of proximal parts of the leaves; (3) 

 that the shoot consists of units, "segments," called phijtons. The second 

 of these theories was later expanded — without the addition of better 



" Forest Products Department, Australian Council for Scientific and Industrial 

 Research Organization. 



