500 EEY. GEOKGE HEN SLOW OX A THEORETICAL 



entering into the sheath. This tendency to "break up" the 

 xylein-cylinder, so to say, is found in many exogens with sheath- 

 ing leaves ; as well as the production of central or medullary 

 cords, which is particularly characteristic of Umbelliferce, e. g. 

 Silaus pratensis*. The structure of the fibro-vascular system 

 of the nodes of stems of umbelliferous plants appears to be 

 particularly significant; for sections through those of fennel f, 

 carrot, &c, show an extraordinary amount of confusion in con- 

 sequence of the departure of large numbers of cords for the 

 leaf-sheath. If sections be made at the node of the aquatic 

 stem of Limnocliaris Humboldtii, or that of a flowering-srem of 

 a Narcissus, a very similar confusion will be seen, certain cords 



isolating themselves, being destined in the latter plant for the 

 spathe. In both cases it may be observed that the nodal con- 

 fusion follows upon regularly arranged zones of fibro-vascular 

 cords in the internode below. 



A further fact follows. In consequence of so large a number 

 of cords entering the petiole, the latter often reveals a struc- 

 ture, as seen in a cross section, identical in appearance with that 

 of an endogenous stem. Such is well seen in Angelica sylvestris, 

 CaltJia palustris, Ranunculus Lingua, Tommasinia Szowitsii, 



Myriophyll 



These tend to form a 



similar nodal plexus, w T hile the cords of the petiole of the first- 

 named plant form a very irregular circle surrounding a lysi- 

 genous lacuna. 



The nodes of the above-mentioned exogens differ from those 

 of endogens in retaining a central medulla, the confusion of 

 the latter spreading all over a transverse section ; but Ranun- 

 culus Ficaria advances a step further in this direction by occasion- 

 ally sending a branch completely across the centre. 



The above-described coincidences in structure between aquatic, 

 umbelliferous, and other exogens and the sheathing bases of 

 well nigh all endogens is, to say the least, highly significant §. 



* 



"Ueber das centrale Gefassbiind el-system einiger Umbelliferen," H. W. 

 Reichart, SB. der mat.-naturwiss. CI. Akad. der Wiss. Wien, xxi. 1856. 



t De Bary does not appear to have seen the " confusion n I speak of; his 

 figure (Vergl. Anat. Vegetationsorg. p. 241) giving no idea of what takes place 

 at the nodes. 



% The last two were obtained from the Botanic Gardens, Cambridge. 



§ After the above paragraph was written I came across the following 

 interesting passage in a letter from M. Mirbel to M. Deleuze, entitled " Con- 

 siderations §ur la maniere d'etudier THistoire naturelle des Yegetaux, servant 



