22 GENERAL DIFFERENTIATION OF THE PLANT-BODY 



been called hair-like among the Thallophyta have the most different origin 

 and the most different functions l they may be fixing organs, they may 

 be protective organs of the most various kinds, for example, secreters of 

 mucilage, and there are many the exact significance of which we do not 

 know. They have this only in common, that they appear as small append- 

 ages of the thallus and have often a certain external resemblance to 

 the hairs of higher plants, but, as there is no epidermis here, they want 

 naturally the technical character which belongs to hairs of higher plants 

 and which, as we have seen above, is regarded as critical. 



The lower plants furnish a starting-point for the determination of 

 the critical marks of organs, inasmuch as they exhibit a series of gradual 

 differentiations of organs in relation to division of labour in their functions, 

 and they do this not only along one line but they repeat it in several 

 different series. The differentiations of members which we find in the 

 higher plants are therefore to be considered as merely special cases of these 

 more general ones. 



UNICELLULAR AND PLURICELLULAR PLANTS, CELL-COLONIES 



AND CELL-DOMINIONS 2 . 



The external relationships of configuration of the bodies of plants are 

 determined by the peculiarities of their living substance, the protoplasm, 

 which in the higher plants is enclosed within the numerous cells which 

 compose the plant ; it is only amongst the lower plants that we find uni- 

 cellular bodies. Sachs 3 has however shown that the traditional idea of 

 the cell is now become inapt and that it leads often to incorrect com- 

 parisons. If we speak of a caulerpa and a diatom or desmid as unicellular 

 we only indicate thereby one external circumstance, namely, that these 

 plants consist of a non-chambered protoplasm invested by a cell-wall 4 . 

 But the inner structure of the protoplasm is very different in the cases 

 cited, and the difference is marked by the presence in the diatom and 



1 See, amongst others, Moebius, Morphologic der haarartigen Organe bei den Algen, in Biolog. 

 Centralblatt, xii. p. 71. 



' 2 [The word dominion is used to convey the meaning of the German ' Staat.' To have 

 employed the literal translation 'cell-state' as the equivalent of the German ' Zellstaat ' might have 

 led to confusion for obvious reasons.] 



3 See especially his Physiologische Notizen : II. Beitrage zur Zellentheorie, in Flora, Ixxv 

 (1892), p. 57; and IX. Weitere Betrachtungen iiber Energiden und Zellen, in Flora, Ixxxi 

 (Ergbd. z. Jahrg. 1895). 



* L. Klein says, for example : ' The highest stage of construction reached by an unicellular 

 individual is found in the Siphonieae ... in which nature shows to what height of development it is 

 possible for a single cell to attain, for this is the character of the thallus in these plants notwithstanding 

 the great amount of division of labour it exhibits.' Unters. iiber Morphologic und Biologic der 

 Fortpflanzung bei der Gattung Volvox, in Ber. d. Naturf.-Ges. zu Freiburg i. B., v (1890), p. 43. 



