DICTYOGENS. 211 



CLASS v.— DICTYOGENS. 



Retosae, Ed. pr. p. 358. (1836).— Dictyogens, Bot. Reg. 1839. Misc. p. 76. 



There is among the plants refeiTed by Jussieu to his Monocotyledons, and 

 consequently by later Botanists to Endogens, a small number of species 

 whose foliage and habit of growth are so very peculiar, that the 

 reference of them to Endogens is wholly dependent upon their conformity 

 in the structure of the embryo. They have a broad net-veined foliage, 

 which usually disarticidates with the stem, and in some cases the small 

 green flowers are very nearly the same as those of such plants as Menisper- 

 mum, among Exogens. For these reasons I have endeavoured to show that 

 they ought to be regarded as a transition class partaking somewhat of the 

 natm-e of Endogens and also of that of Exogens. And if we regard 

 merely the foliage, the distinction seems admissible, for no Endogens possess 

 such a character except a few Arads, otherwise widely different. The 

 nearest approach to this structure, with which I am acquainted, occurs in 

 Lilium giganteum, but the leaves of that plant have a flat foliaceous petiole 

 and do not disarticulate. The broad-leaved Amaiylhds like Griffinia, 

 Eurycles, &c., are totally different ; their leaves not only having no articu- 

 lation with the stem, but having no reticulations between the ribs, further 

 than what arises from the anastomosing of the fine parallel secondary 

 veins which connect them. 



It is not, however, in the leaves alone that a distinction is found between 

 Endogens and Dictyogens. If the annual branches of a Smilax are exa- 

 mined, there is nothing indeed in their internal structure at variance with that 

 of a stem of Asparagus ; they are exactly Endogenous ; but in the rhizome 

 of the whole genus (take the SarsapariUa of the shops for instance) the 

 wood is disposed in a compact circle, below a cortical integument, and 

 surrounding a true pith ; in that of Smilax aspera the woody matter is 

 disposed in the form of a cylinder, inclosing a centre of soft cellular matter ; 

 and the vessels of the cylinder have an evident tendency to arrange themselves 

 in hues fomiing rays from the centre. In Dioscorea alata the stem itself is 

 formed of eight fibrovascular wedges placed in pairs, with their backs touch- 

 ing the bark, surrounding a central pith and having wide medullary plates 

 between them ; in fact, when the stems of this plant are in a state of decay, 

 the eight fibrovascidar wedges may be pulled asunder, like those of a Birth- 

 wort or a Menisperm. In the curious Testudinaria elephantipes the 

 structure of the stem is of nearly the same kind ; several bundles of 

 fibrovascular tissue form a circle surrounding a pith, and pierced with broad 

 medullary processes. Lapageria and Philesia have each a zone of wood 

 below their bark, and a central pith in which the common fibrovascular 

 bundles of Endogens are disposed ; a tendency to which is also observable 

 in Smilax. It therefore seems that the peculiarities in the foHage of these 

 plants are accompanied by others equally remarkable in the structure of 

 the stem ; indeed I do not see why the stem of a common Yam has not as 

 good a title to be regarded Exogenous as Endogenous. Schleiden 

 indeed has remarked that he believes it to be the regular structure of 

 the roots of Endogens to have a simple circle of fibrovascular closed 

 bundles ; and this seems to be sometimes the case. But I do not find it 



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