ORIGIN OF EXDOOEXS FROM EXOGEXS. 509 



number of genera of Dicotyledons and Gymnosperms ; but 

 unfortunately makes no mention of aquatic plants. As far 



as 



negative evidence goes, this may be due to the fact that 

 lie did not find it in any such plant, as he says was the case with 

 all endogens. Now we know that the mechanical supportive 

 tissues all become degraded or vanish in water-plants : so the 

 absence of any " reseau de soutien " would be exactly what one 

 would a priori expect ; and if my contention be a true one, then 

 a like absence of it from endogens can be accounted for. With 

 regard to this apparatus, he says that it is of a precocious nature 

 and lasts but a short time. It is differentiated at a short distance 

 from the extremity of the root and exfoliates later with the cork, 

 of which it forms a part. It occurs either upon the endoderm 

 or is subepidermal, or else at some intermediate position in the 

 cortex. It is formed by the cells being thickened in various 

 patterns on the interior and radial walls. 



Of course the significance of the above-mentioned observations 

 lies in the fact that the Nymphceacece and other water-plants, &c, 

 agreeing with endogens in so many points afford accumulative 

 coincidences which collectively support a very high probability 

 of the direct descent of existing endogens from aquatic exogens. 



Dimensions of Vessels. — M. de Bary records another point 

 in which Nymphwacce agree with endogens generally, namely, in 

 the size of the vessels of the xylem in the roots. Describing 

 those of endogens, he says : — " The xylem-bands, consisting of one 

 or a few rows of elements, usually begin at the periphery with a 

 short uninterrupted radial band of narrow tracheae, which become 

 gradually wider towards the inside. These are suddenly followed 

 in the centripetal direction by one or a few very wide, reticu- 

 lated or pitted vessels " *. A rationale of this fact may be that, 

 being submerged plants (as are the Nymphceacece, and theoreti- 

 cally all endogens were so originally), larger vessels are required 

 for the flow of a superabundance of water; just as large or 

 several water-pores are often found at the ends of submerged 

 leaves, as in CallitricTie. With such, indeed, sometimes the 

 tissue surrounding them actually breaks down, leaving a large 

 hole instead, as occurs in Myriophyllum.&c.t 



* Comp. Anat. &c. p. 357. 



t An analogous interpretation may perhaps explain the occurrence of large 

 vessels in the earliest formed wood of certain trees, as of the oak, ash, and 

 Castania vesca. These trees are very late in putting on their foliage, conse- 



