Homology presents a more difficult problem, for in plants there may be various degrees of that closer correspondence of parts connoted by the term. The strictest conception of homology is that designated homogeny, which may be defined as including those structures which are genetically related in so far as they have a single representative in a common ancestor (Lankester). This implies repetition of an individual part bearing a definite relation to the whole organism. In plants with their continued embryology such correspondence is rarely possible, owing to the indefinite number of appendages produced. And yet the leaves of a shoot are comparable in other respects; but their homology can only be recognized in a less stringent sense than homogeny. Further, it may be held as doubtful whether all such parts as those designated "leaves" represent any essential plan of con struction applicable to plants generally. The question arises whether leaf-like organs may not have originated independently in distinct phyletic lines : for instance, among the higher Algae, the Bryophytes, and the vascular plants. These diverse leaf-like organs may rather be regarded as being based upon such uniform ity of results as has been achieved polyphyletically in descent, that is, by various types of organisms independently. In that case their likeness would be described as homoplastic (Lankester). Limiting Factors.—Given a sufficient nutritive supply, con tinued growth of the plant-body may bring the organism up against certain limiting factors, of which the most obvious are concerned with mechanical stability, and the preservation of a due balance between surface and bulk, whether of the external contours, or of the internal tissue-surfaces. The size-factor may thus dominate not only the possible dimensions, but also the form of the plant-body and of its constituent tissues. The under lying principle is that of "similar structures," long ago recognized by Galileo, and familiar to architects and engineers. Applying it mechanically the strength of a structure varies as the square of the linear dimensions, and the mass or weight as the cube. Any plant endowed with continued growth, and with branching, is con stantly approaching a size-limit defined by mass versus strength. Somewhat more than 3ooft. is the extreme practicable height for a tree with the usual proportions and type of construction of the trunk, branches and twigs. A higher trunk would bend or break under the stress of wind.
The size-factor is also an effective influence on form ; this is seen in respect of the proportion of surface to bulk. If the same form is maintained in an enlarging body, its surface increases as the square, while the bulk increases as the cube of the linear di mensions. Since nutrition and transit of materials are dependent upon surface action, the maintenance of a due proportion of sur face to bulk in an enlarging organism may become critical. Its effect may be traced externally in the subdivision of submerged leaves, or indeed in foliar development itself : or it may appear internally in the complicated segregation of the vascular tracts in plants where cambium is absent or sluggish, as in the ferns or Lycopods. Such influences of the size-factor have long been recognized by animal morphologists : they are now beginning to find their place in the morphology of plants. (See F. 0. Bower, Proc. Roy. Soc. Edin., vol. xli., xliii.) Causal Morphology.—The influence of the size-factor may be offered as an example of causal morphology, that is, the relation of form to environmental influence. It must suffice here merely
to mention those better known branches of its experimental study which deal with the relation of form to such influences as gravity and light. Hofmeister introduced experiment in this field in his Allgemeine Morphologie der Gewachse (1868) as a corrective to the idealistic morphology of the previous period. His observations dealing with the arrangement and final form of the parts consti tuting the shoot were the natural precursors of the later work of Sachs, who recognized that the form and manner of life of plants "must in great part have arisen through the perpetual operation of gravity and light." (V orlesungen, ate Aufl: p. 545.) Adaptation.—The scalyness or succulence of isolated species or genera under xerophytic conditions may be taken as examples of protective adaptation. Such characters are often hereditary : for instance, the succulence of a cactus or of a spurge appears in the seedling, even when grown in moist air: a seedling parasite, such as the dodder, produces parasitic seedlings that require at tachment to the host for full development. Here the germ cell carries the character held as adaptive. On the other hand many formal characters that are specially related to climate or soil may be less marked, or even absent in the offspring grown under medium conditions. They have not been permanently stamped upon the germ-cells. Such differences raise the general question of the inheritance of adaptive form.
Inheritance of Adaptive Characters.—Direct experiments that adaptive or impressed characters may be inherited in plants have so far given indecisive or negative results. All such experi ments necessarily range within the narrow limits of laboratory time. If, however, a sequence of events conducted with the lati tude of geological time gives a positive result, it seems right to give such a positive conclusion precedence over observations limited to a brief period. More than one good case has recently been brought forward showing the establishment of an ontogenetic adjustment as a permanently inherited character, where a consecu tive evolutionary sequence of steps in its appearance can be traced from early geological periods, and confirmed by comparison of living species and genera. (See F. 0. Bower, Ferns, vol. iii., Cambridge, 1928.) The conclusion follows that, in plants at least, direct ontogenetic adjustments, if repeatedly imposed, may be come hereditary. Other opinions are held on this point, especially by zoologists (see LAMARCKISM, HEREDITY). If such inheritance were wide-spread or general, an important factor would be sup plied in explanation of the prevalent phenomenon of adaptation.
But it would not provide a full explanation. The nature of the organism is the more decisive factor in adaptation. It suggests a tendency of initiative in the organism itself, which would go far to explain certain broad features in evolution. Intensive study of the coherent class of the ferns has shown similar trends of advance in distinct phyla-parallel changes which constitute together a positive phyletic drift. This may be held as determined by the "nature of the organism": and to it, acting on differences in that nature, may be attributed the results which we see worked out in those distinct classes and divisions of the vegetable kingdom, which nevertheless may have evolved under conditions essentially alike.