ORIGIN OF ENDOGENS FROM EXOGE.VS. 489 



either due to a community of descent and represent affinity, or 

 to similarly adaptive processes. In the ease of the two great 

 classes of Phanerogams, Exogens and Endogens, notwithstanding 

 their great and well-known differences, there are several fea- 

 tures which they have in common which point to the termer 

 rather than the latter. In looking for proofs of a community of 

 descent, one turns always towards the least differentiated struc- 

 tures ; and we find them, therefore, in the anatomy of many of the 

 main roots, in the morphology of the stems of seedlings, in peduncles 

 and pedicels, as well as in the floral organs. In the simplest 

 form of roots the arrangement of the elements is exactly the 

 same, each being equally pronounced. Indeed, we may include 

 those of Cryptogams, such as ferns. M. Ph. van Tieghem has 

 described and figured several in his paper on the symmetry of 

 plants # ; so that, by looking at his illustrations alone, it would 

 be impossible to say to which class each belonged. 



Perhaps the most marked distinction between the two classes 

 lies in the structure of the woody stems ; but in such simple organs 

 as pedicels a common arrangement is to have a definite number 

 of cords arranged symmetrically in a circle in both classes, the 

 number only being different. Thus, in a crocus there are six, 

 while a primrose has five. Again, it* the flower-stalk of Anemone 

 coronaria be compared with that of a daffodil, no appreciable 

 difference is perceptible between them. They both have two 

 ovals of separate cords arranged around a lacuna in the middle. 



A similar simplicity is ofren seen in the first year's growth 

 of herbaceous plants, whenever the fibro-vascular cords are all 

 isolated in a circle round the central medulla. Interfascicular 

 cambium may occur and subsequently fill up the interstices, 

 thereby making a complete zone of xylein. We then get the 

 typical exogenous structure. In the case of endogens the 



o 



characteristic difference likewise takes place at the same stage, 

 in that the subsequent cords are separate, but are now scat- 

 tered throughout the fundamental tissue. Consequently the 

 cortex, medullary rays, and medulla are not isolated as the cords 

 are developed. The fibro-vascular cords remain, however, oriented 

 as before, the cambium being arrested. 



Now this is just what one finds to be the case, in an incipient 

 degree, in an amphibious plant, such as Bidens cernua. In the 



* KW1 



Symmetric de Structure cles Plantes," Ann. des Sci. Nat., Bot. 5 st-r. xiii. 

 P- 5, 1870 ; also ■ Traite de Botaniquc,' vol. i. p. 075. 



