232 



PRINCIPLES OF PLANT-TERATOLOGY. 



be noted that they are distichous and with very pro- 

 nounced sheathing bases. 



Again, cases have already been cited where, in 

 Dicotyledons, seedlings with a single cotyledon have 

 exhibited alternate phyllotaxis on the shoot above. 



On shoots in which alternate phyllotaxis abnormally 

 appears it does so usually in the higher part of the 

 shoot, the lower nodes tending to form the normal 

 pairs of leaves ; but this is not invariably the case 



The change from opposite-decussate to spiral phyllo- 

 taxis which occurs in such cases as those cited in 

 Buddleia and Lonieera is a very imperfect one. Much 

 more, but still very far from completely, perfect are 

 the reversions exhibited by the contorted stems of 

 carnation, teasel, etc. ; in the carnation it was observed 

 that the apical bud (Plate XXI) showed the leaves 

 neatly arranged in a spiral manner ; the torsion arises 

 as soon as the internodes begin to lengthen. The 

 same torsion has been observed in some species of 

 horse-tail (Equisehtm) in which cases it must represent 

 a reversion to alternate phyllotaxis obtaining in 

 ancestors more remote than those we know of in the 

 Carboniferous and Devonian Periods; and it is decidedly 

 of interest that Heer found exactly the same type of 

 torsion in certain fossil Equisetaceae. 



Spiral phyllotaxis can be induced experimentally in 

 plants Avhich normally exhibit the opposite-decussate 

 type ; if steins of the horse-chestnut (JEscnlus Hlppo- 

 castanum) be cut down, the shoots which subsequently 

 spring up bear leaves arranged according to the f type 

 of phyllotaxis ; the luxuriant sucker-growth would 

 seem to cause a reversion to a more primitive type of 

 leaf -arrangement. This view is strongly supported by 

 the fact that if stems of Piwis, especially P. Pinea, be 

 lopped, the short-shoots proliferate and bear primordial 

 leaves {otherwise produced only in the seedling) spirally 

 arranged on the axis. Both of these may be regarded 

 as cases of traumatic reversion. 



In connection with the generally admitted view 



