392 CONDUCTING SYSTEM 



phylogenetic development. The author has demonstrated the procambial 

 origin of the endodermal layers in the Juncaceae and Cyperaceae, and 

 a similar mode of development probably prevails among Grasses, except 

 that in certain cases the endodermis represents a partially modified 

 parenchymatous bundle-sheath. Among Ferns the endodermis originates 

 from fundamental meristem. In this connection, Eussow states that 

 elements of the ground-meristem surrounding a procambial strand under- 

 go one or more tangential divisions, thus giving rise to radial rows of cells. 

 Either the outermost of the resulting layers, or one nearer the middle, 

 is converted into the endodermis, while the inner layers give rise to the 

 above-mentioned parenchymatous sheath (inner phloeoterma of Stras- 

 burger). In the smaller bundles of Ferns, the genetic relation of the 

 endodermis to the layers immediately interior to it is clearly recognisable 

 even in the adult condition. In roots, finally, the endodermis arises 

 from the layer of fundamental meristem which immediately adjoins the 

 axile strand. 



Since the meristematic layer from which the endodermis originates, 

 does not always become bodily transformed into endodermal cells, it 

 cannot be simply regarded as the young endodermis. The author, 

 therefore, proposes to apply the term coleogen to this layer, and further 

 to distinguish, according to the morphological character of the layer in 

 question, between procambial and fundamental coleogen. Just as the 

 principal function of the phellogen is the production of cork, so the 

 principal function of the coleogen is the formation of an endodermal 

 sheath, as indeed its name implies. But other tissues may also arise, 

 in part at least, from coleogen. As already indicated, this point is 

 demonstrated by the development of the vascular bundles of Ferns, 

 where the fundamental coleogen surrounding the procambial strand 

 produces parenchymatous layers in addition to the actual endodermis. 

 In the leaf of Scirpus Holoschocnus the procambial coleogen undergoes 

 tangential division at points corresponding to the positions of the 

 future hadrome- and leptome-groups, the innermost of the resulting 

 segments developing into genuine bast-fibres ; in this case, therefore, 

 the coleogen takes part in the construction of the mechanical system. 



Passing on to consider the mode of origin of the individual con- 

 ducting strands, we may first discuss what can be made out by means 

 of transverse sections. It will be found that the several elements of a 

 strand do not arise simultaneously, but that their differentiation is, 

 on the contrary, successive. As a rule, the organisation of permanent 

 conducting elements starts at the margin of the strand and gradually 

 advances towards its centre. By a slight modification of Eussow's ter- 

 minology, we arrive at the names of protohadrome and protoleptome for 

 these first-formed vascular elements. 



