founded on the Structure of their Seedlings . 77 
Nyctagineae. 
Abronia umbellata .... Darwin, 10 , p. 95 (Fig. 61). 
Klebs, 29 , p. 561 (Fig. 10). 
A. arenaria Klebs, 29 , p. 560. 
A. grandiflora Klebs, 29 , p. 560. 
The existence of some Dicotyledons with only one seed- 
leaf is commonly explained by the supposition that one 
seed-leaf of the pair is abortive. This may be so in some 
of the species in the foregoing list, as Abronia umbellata (see 
Darwin, 10 , p. 95) : other cases may arise through the fusion 
of cotyledons by one or both margins. 
3. Origin of the Monocotyledonous Habit. 
If the homology of the single seed-leaf in Monocotyledons 
with both the seed-leaves of Dicotyledons be accepted as 
a working hypothesis, we are at once confronted with another 
question. How did that fusion begin, and of what ad- 
vantage was it to the ancestral Monocotyledons in which it 
became stereotyped ? That the union of seed-leaves does 
offer advantages to seedlings under certain conditions is clearly 
shown by the existence of a number of Dicotyledonous species 
in which they are normally united for a great part of their 
length. 
Comparison of the species mentioned in Table I with each 
other shows that they have another character in common 
besides the possession of a cotyledonary tube. With one 
exception their hypocotyl is always much reduced in length, 
and is commonly thickened. As a rule the first internodes 
of the plumular axis are likewise more or less completely 
suppressed. Rhizophora is said to have an elongated hypo- 
cotyl, but the conditions under which it grows in tropical 
swamps are unique, and we cannot be surprised by exceptional 
adaptations to them. 
The great majority of the species mentioned are tuberous : 
the others (. Podophyllum , Serratula, Polygonum , Rheum) form 
