10 PLANTS AND MAN 



of labor among the cells of a multicellular body brings about a 

 more efficient carrying on of all the life functions; a structure 

 designed for protective purposes, for example, can act as such 

 far better than another which is fitted for absorption of water. 

 In the evolution of multicellular plants, some groups of cells 

 specialized in protective duties, others absorbed water or gases, 

 still others assumed the responsibility of manufacturing food. A 

 large group of cells having a similar structure and carrying on 

 similar special functions is known as a tissue, but although the 

 bodies of plants and animals may thus be built up of numerous 

 tissues, the cell remains the unit of structure and function. The 

 activities of the organism as a whole is merely the sum total of 

 all the activities of the individual cells. All of the higher groups 

 of animals and plants are built up of tissues; examples of such 

 tissues among animals are muscles, nerves, glands and bones. In 

 the plant kingdom there are a few main types of tissues (fig. 3). 

 The outer protective cells of leaves, flowers and fruits is epidermal 

 tissue; parenchyma tissue of relatively unspecialized cells makes 

 up the bulk of leaves, flowers, fruits and young stems; vascular 

 tissues conduct water and the minerals and foods dissolved in it; 

 supporting tissues act as skeletal elements. In full-grown woody 

 plants vascular and supporting tissues comprise the bulk of the 

 roots, trunks and branches. 



Many of the lower plant and animal groups include organisms 

 which have not gone far beyond the tissue stage of body organiza- 

 tion. Such are the simpler marine invertebrates — the corals and 

 sponges. In the plant kingdom this condition is the rule among 

 the pond-scums and seaweeds (algae), and among the mush- 

 rooms and molds (fungi). A plant body characterized by only 

 a few different tissues co-ordinated in a primitive fashion is 

 known as a thallus; for this reason the algae and the fungi are 

 often referred to as thallus plants or thallophytes (fig. 4). 



As division of labor proved to be a successful evolutionary 

 advance, specialization progressed beyond the union of cells 

 into tissues. Groups of tissues with some organic connection and 

 concerned with the same general function formed greater cell 

 aggregates known as organs. Familiar organs in the animal 

 kingdom are the heart, lungs and stomach. Plants also have ad- 



