DISTINCTION BETWEEN MEMBERS AND ORGANS. I51 



generally appendages which may be termed hairs ; but there are never any roots 

 in the morphological sense, and the term leaf, as understood in higher plants, can no 

 longer be rightly applied even in those cases where the external form of the mature 

 parts is similar to the foliage-leaves of higher plants, e.g. Laminaria digitata, &c. 

 It is now agreed to apply to those vegetable structures in which the morphological 

 distinction of stem and leaves cannot be carried out in the present state of our 

 knowledge (and which have never any true roots), the morphological term Thallus 

 or Thallome. In contradistinction to Thallophytes, all plants in which leaves 

 can be morphologically distinguished might be termed Phyllophytes ; the name 

 Corviophytes has, however, been given in preference to them. From what has 

 been said it will be seen that the thallophyte is only distinguished from a cormophyte 

 by the lateral outgrowths which occur somewhere or other on it not presenting 

 sufficient morphological distinctions from the part which bears them, to permit us to 

 term them leaves in the same sense as in the more highly differentiated plants. 

 But as the morphological distinctions of stem and leaf are not yet sufficiently 

 established even in higher plants, it is impossible to draw a sharp boundary 

 between Thallophytes and Cormophytes, and indeed it is certain that one does not 

 exist. 



If we now accept the terms Thallome, Stem (Caulome), Leaf (Phyllome), and 

 Hair (Trichome)\ in the senses indicated, it can no longer be said that the leaf is the 

 organ for this or that function ; for leaves may undertake all possible functions ; and 

 the same remark applies also to the other parts. It is therefore on all accounts 

 inexpedient simply to apply the term Organs to thallomes, stems, leaves, and hairs, 

 for many of them have in fact no function at all. In order to avoid this mode of 

 expression, which is confusing and foreign to morphology, it is obviously best to 

 speak in this sense not of Organs, but of Members. The term Member is used when 

 we speak of a part of a structure in reference to its form or position, and not to any 

 special purpose it may serve. Thus, from a morphological point of view, stems, 

 leaves, hairs, roots', thallus-branches, are simply members of the plant-form ; but a 

 particular leaf^ a particular portion of the stem, &c., may be an organ for this or that 

 function, which it is the province of physiology to investigate. 



The morphological nature of a member is best recognised in its earliest 

 stages of development, and by its relative position in the series of processes of 

 growth ; the morphological definitions depend therefore essentially on the history 

 of development. 



The older a member becomes, the more obvious becomes its adaptation to a 

 definite function, the more completely is its morphological character often lost. In 

 their earliest states the members to which the same morphological term is applied 

 {e.g. all the leaves of a plant) are extremely similar to one another; at a subsequent 

 period all those distinctions aj-ise which correspond to their different functions. 

 We can now arrive at a definition of Metamorphosis which may be used in a 

 scientific manner : — Metamorphosis is the varied development of members of the 

 same morphological value resulting from their adaptation to definite functions. 



\ 



^ See Nageli und Schwendener, Das Mikioskop, vol. II. p. 591. 



