130 EXTERNAL CONFORMATION OF PLANTS. 



This mode of investigation, applied to Vascular Cryptogams and Phanerogams, 

 has shown that all the organs of these plants may be referred to one of these 

 morphological categories ; every organ is either Stem (Axis), Root, Leaf, or Hair. 

 Mosses have no root in a morphological sense, although they possess organs 

 which completely fulfil the functions of roots ; on the other hand most Mosses have 

 leaves which grow on stems (axes). In Algae, Fungi, and Lichens, the body of 

 the plant has generally appendages which may be termed hairs ; but roots in the 

 morphological sense are ahvays absent, and the idea of the 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. Lanmiaria digitala, &c.). It is now agreed to apply to those vegetable struc- 

 tures in which the morphological distinction of stem and leaves cannot be carried 

 out in the present state of our knowledge (and from which true roots are always 

 absent), the morphological term Thallus or Thallome. In contradistinction to 

 Thallus-plants (Thallophytes), all plants in which leaves can be morphologically 

 distinguished might be termed Phyllophytes ; the name Cormophytes 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 distinc- 

 tions 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 dis- 

 tinctions 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 now we accept the ideas 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 call the thallomes, stems, leaves, and hairs organs, 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 whole in reference to its form or position and not to any special 

 purpose it may serve. In the same manner, 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 develop- 

 ment. 



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 



Cf. Nageli und Schwendener, Das Mikroskop, II. p. 591. 



