DISTRIBUTION OF PLANTS ((IN.).
Physiological ecology. whether anatomical or organographic, is concerned chiefly with the origin and meaning of the tissues and organs of plants. In the interpretation of this field there are two prominent theories, which may he briefly outlined. The older and commoner theory is based 4)11 teleological assumptions, and may be called the theory of purpose or design. The theory of design has also hail two aspects: (1) That in which rigid special creations were postulated in order to account for the relations between structures and functions: (2) that in which natural selection. acting for many gen erations. was supposed to he the cause of special ized structures obviously adapted for certain ends. The newer and less minimal theory is based on dysteleological assumptions. and may be called the accident or gall theory. Plant structures are regarded as the result of various purely physical and chemical forces: if a struc ture is of value, it is but a matter of accident. Just as a gall is regarded by every one as the result of forces set in operation by the insect's sting. so all organs and tissues are the inevita ble result of mechanical agents. Possibly the correct standpoint is an intermediate one. Struc tures cannot be built, nor functions carried on. contrary to chemical and physical laws; hut within certain limits there is perhaps room for successful variation, and hence for natural selection and the gradual improvement of struc tures in relation to plant functions. Some of the arguments for natural selection and the teleological view are found in the structure and mechanics of stomata, which are the opposite of what one would theoretically expect : the stor age of water or reserve foods is a phenomenon hard to account for without natural selection. Experiments show that some plants are rigid and some plastic; rigid plants have commonly lived for ages in rigid habitats, as is particu larly true of desert plants. Plastic plants, on
the other hand, often live in variable habitats, the best examples of all plastic plants being the amphibious species. It would seem as if per manence of habitat causes the development of rigid structures. as is well shown in the Ile gonia, which has remained xerophytic in struc ture through generations of mesophytic culture --i.e. culture in moist surroundings. And it would seem as if the marvelous plasticity of such plants as Itanuneulus. Nasturtium. Polyg onum. Proserpinaca, Shim. etc.. is a result of long life in variable habitats. The desert plant shows the selection of adapted but rigid forms, while the amphibious plant shows the selection of the capacity to vary, a far subtler type of selection. While these argu ments favor the natural selection and teleolog ical theories. current dysteleological views have certainly clarified the atmosphere. The extreme views of some teleologists. who see a purpose in cverything. rust surely be discarded; but it re mains for the future to strike a proper balance between the design and accident theories in reologv. For the present. therefore, the ecolo gist should strive to a seertain the influence of external agents on plant form. and should also seek to discover the advantages possessed by eertain in the performative of the plant functions, but should have the determination of the exact relation between form am l func tion to future investigation.
The subject matter of anatomical ecology is not presented as such in this work. The purely 1110 rp 11010g ill] I aspects are treated under ANAT OMY OF PLANTS. Anatomical matter is also pre sented in connection with the y1)10 1•111"TE: XERoPHYTE; LEAF: IZooT: SYMBIOSIS Organographie ecology will be treated in neetion with the various organs of plants I ,ee