ORIGIN OF EN7)0GENS FROM EXOGENS. 527 



occur occasionally in exogens, e.g. Berberis and Rumex ; for every 

 portion of the spiral of the latter series which, when projected 

 upon a plane, forms a circle (excepting the case of £) contains 

 four leaves, just as a similar circle of the former series contains 

 three. I am not aware, however, of the secondary series occur- 

 ring in endogens at all; the leaves of Potamogeton, Cyclanthus, 

 &c, e. g., being distichous, or \. 



If, however, endogens be derivable from an early stock with a 

 dicotyledonous embryo, then it would not be very surprising to 

 find occasional reversion to what prevails in exogens, especially 

 in the most elementary, simple, or degraded parts. 



Now, M. Ph. van Tieghein has found, as explained above *, 

 that when Asparagus germinates there is actually a pair of 

 leaves developed at right angles to the plane passing through the 

 one cotyledon and the first leaf, which now takes a position oppo- 

 site to it. These two pairs subsequently give rise to the f arrange- 

 ment, as I have elsewhere explained. Hence fours, as in the 

 flowers of Potamogeton, seem probably referable to a reversion to 

 a very primitive coudition of an opposite and decussate arrange- 

 ment, now only to be seen elsewhere in the germinating plantlet. 

 This, at least, is the only interpretation I can suggest ; for 

 although M. Ph. van Tieghem found it to be as above described in 

 Asparagus, one would hardly expect that this is an isolated con- 

 dition of things in endogens. Indeed, a similar feature appears 

 to be observable in Tamus. 



Conclusion". — That endogens and exogens have had some 



h ViiW "^~ ~^~fc> 



common origin is, in the first place, obvious from the numerous 

 points of agreement between their organs and tissues generally, 

 as shown, e. g., in the structure of their flowers and anatomy of 

 their roots, &c. ; but the object of the present paper was to prove, 

 if possible, that there was a more special connection between 

 endogens and an aquatic habit. 



The coincidences in points of structure between endogens 

 generally and aquatic, subaquatic, as well as such exogens as 

 may reasonably be supposed to have had an aquatic ancestry, 

 will have been now seen to be perhaps more numerous than the 

 points of agreement with exogens generally. Though it may be 

 impossible to verify all the inferences by experiment, yet nature 

 supplies us with several " experimental verifications " in the 



Supra, p. 492. 



LINN. JOURN. — BOTANT, TOL. XXIX. 2 Q 



