254 Wilson . — The Mucilage - and other 
and constitution, and bear abundant mucilage-glands, at 
which time the distinctive feature was transmitted to the 
seedling ? Or might not the presence of the glands lead one 
to surmise that these three genera might have had a common 
ancestry, this character lingering distinctly in the seedling 
although almost entirely lost now in the adult Acantholimon ? 
If they are not present, the problem becomes more complicated. 
Their invariable occurrence in the cotyledons of Armerias and 
Statices may safely be presumed, and they indicate very 
markedly the essential uniformity of structure and habit of 
the two genera. Their absence from the cotyledons of 
Plumbago opens up a wide field of speculation. It would be 
vain at present to attempt to elucidate the intricate and far- 
reaching problems in embryogeny which confront us in 
connection with the presence or absence of the cotyledonary 
mucilage-glands. A much fuller enquiry must first be made 
with other genera and species. The causal conditions which 
lead to the acquirement of the glands in the axils of the 
leaves lead also to their appearance in the cotyledons. The 
vicissitudes undergone by the parent and seedling are 
essentially the same. The occurrence of the glands in the 
cotyledons of Armeria and Statice certainly points to some 
occult and exceedingly important function which the mucilage 
performs in the economy of the species. 
A considerable variety of plants having affinity with the 
Plumbagineae have been studied, with the object of ascer- 
taining whether the latter alone possess the forms of glands 
above described. The glands of Frankeniaceae and Tamaris- 
cineae already described by Volkens 1 most nearly approach 
them, the latter especially bearing, in respect of both form 
and function, a marked resemblance to chalk-secreting 
Mettenian glands. The Frankeniaceae are recognised to be 
related, although remotely, to the Plumbagineae 2 , and it is a 
1 Volkens, Flora d. ^Egyp.-Arab. Wiiste, 1887, Taf. v, Figs. 12, 13, and 
8,9. 
2 Le Maout and Decaisne, Traite general de botaniqne, Eng. ed. 1876, pp. 527, 
253, 264. 
