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T907] CHRYSIER—POTAMOGETONACEAE 175 



CYMODOCEA 



This genus shows several points of resemblance to Zostercae, e. 





the filamentous pollen, elongated creeping stem, frequently ribbon- 

 shaped leaves, and, as will be seen, its vascular structure is closely 



related. 



The best general account is that by Bornet (2) of the 

 species C nodosa, though the account of the vascular anatomy is 

 meager. Sauvageau has described certain features of the stem and 

 leaf structure of a number of species (24). The only species acces- 

 sible to me has been C, manatoruniy and to it the following account 



applies- 



ih 



cortex which is characteristic of the genus. An outer zone free from 

 lacunae contains near its inner limit a broken circle of cortical bundles, 

 and surrounds an inner zone which is lacunar. The circle of bundles 

 really consists of two sets, an arc-shaped group of four or five on each 

 side of the central cylinder, w^hich is somewhat flattened. Each of 

 these groups corresponds to one of the two cortical strands of Zostera 

 marina, and to the two to four strands of Z. MiieUeri and Z. tasmanka. 

 They are, as would be expected from analogy, connected with the 

 lateral traces of the leaf, while the median leaf trace enters the central 

 cylinder directly. At the node the cortical bundles of each side anas- 

 tomose and send a branch which joins the central cylinder at the point 

 of entrance of the median trace. Certain of these features are shown 

 ^^ fS' 3^' As Sauvageau figures for C serriilata, the central cylinder 

 consists of tw^o bundles lying in the line which passes through the base 

 of the leaf {jig. ji) ; the phloem contains several large sieve tubes, and 

 the xylem of the two bundles is fused and is represented by a central 

 lacuna. As the node is approached, tracheary tissue fills the lacuna, 

 and at the point of entrance of the leaf trace that bundle of the stele 

 lying toward the trace becomes amphivasal, quickly followed by the 

 other bundle. After the entrance of the two bundles from the cortical 

 system three amphivasal bundles are present in the stele, soon becom- 

 ing two and changing to the collateral form as their xylem disappears 

 and is replaced by the central lacuna. The cortical system of Cymo- 

 docea seems to be an extension of the plan seen in Zostera marina, Z. 

 Muelleri forming an easv transition. The jjcnus is to be regarded as 



a specialized one, judging from the anatomy and the filamentous 

 pollen. 



