THE LEAF. 



231 



other orders all the leaves are alternate, the influence 

 from above being entirely dominant over that from 

 below.* It is significant in this connection that in 

 Monocotyledons, where there is only one cotyledon, 

 opposite-decussate phyllotaxis is excessively rare, occur- 

 ring only in certain species of yam (Dioscorea) ; it may 

 be only a coincidence, but the Dioscoreacese is one 

 of the two orders of this class (the other being the 

 Commelynacese) in which the cotyledon is not precisely 

 terminal, and the prominent development of the sheath 

 on the side of the stem opposite to that on which the 

 cotyledon-lamina is developed, along with the very 

 early appearance of the shoot-apex, has caused some 

 botanists (e.g. Dutrochet and Beccari) to suppose two 

 cotyledons to be present. It seems that we have to 

 do here with a structure intermediate between one 

 cotyledon and two : hence the strongly saucer-shaped 

 cotyledon enclosing the plumule. In fact, it is probably 

 a comparable structure to the terminal foliage-leaf of 

 Buddleid previously described whose basal pocket really 

 represents an imperfectly-developed second leaf. This 

 transitional feature of the seedling would only be a 

 natural concomitant of the other features in Dios- 

 coreacese which show them to be transitional between 

 Dicotyledons and Monocotyledons. 



It is probably this character of the seedling which 

 causes the frequent appearance of opposite leaves in the 

 order. It is true that in the Commelynacea3, in which, 

 according to Solms-Laubach, a similar feature of the 

 seedling occurs, the leaves are alternate ; but it is to 



* It may often happen that a reversion to alternate phyllotaxis occurs in 

 a portion of the vegetative stem while the opposite type still holds good in 

 the inflorescence above. In that particular plant, therefore, no influence from 

 the inflorescence inducing alternate phyllotaxis could possibly obtain; never- 

 theless, this influence must be regarded as present in the plant as part of 

 its inherited constitution, for in other members of the genus, order, or 

 cohort the bracts of the inflorescence will be found to be alternately 

 arranged. Once the opposite-decussate arrangement has been adopted by 

 the inflorescence it would appear to be less capable of reverting than is the 

 case with the vegetative axis. Again, for some orders, e g. the Urticaceae, 

 the opposite-decussate type may be the more primitive, so that the first year's 

 seedlings of the elm, for example, would be producing the original type, 

 the alternate arrangement in this genus being the derived one. 



