256 de Fr nine . — The Morphology and Anatomy of the 
tive structure. From such a form, by the embedding of the gland in 
the leaf tissue and the consequent necessary modification of the stalk- 
cells, such a type of mucilage gland as occurs in Aegialitis annulata 1 is 
arrived at. By the limitation of the number of secreting cells to eight, 
and the consequent modification and reduction of the stalk cells to the 
four subsidiary cells , 2 the type of Mettenian gland characteristic of S. 
binervosa , &c., is obtained, in which the secreted substance is water and 
not mucilage. Finally, from such glands, the various modifications found 
in the chalk glands of such species as Limoniastrum monopetalum can 
be readily derived. 
Two views have been put forward as to the working mechanism of 
the Mettenian gland. According to Licopoli and Maury the product of 
secretion is amassed in a space which results from the separation of the 
four internal glandular cells, and it is rejected by the tension of the cells, 
which, however, always remain joined by their lower parts. This view 
appears to be incompatible with the structure of the gland as described 
by later workers. 
de Bary, Volkens, and Woronin all agree that the case is one of 
simple osmotic phenomena, and Volkens states that the glands act as 
valves which become efficaceous as soon as the transpiration of the aerial 
organs is in excess of the absorption of water by the roots. 
The number of glands present on the leaves of the various forms is 
shown in Table I. The portion of the leaf chosen was, in each case, 
the broadest region of the blade, and the numbers in each case repre- 
sent the average of a number of counts. 
Table I. Average Number of Mettenian Glands per scp mm. of Surface. 
Upper surface Lower surface 
of leaf. of leaf. 
S. bellidifolia 9-6 8*3 
S. binervosa , ‘ broad-leaved ’ . . . 8*3 9*6 
Dwarf Form. S. binervosa : 
/mud plant 7-5 12*4 
sand plant 9*6 11*7 
culture from seed 6-9 9-6 
narrow-leaved lateral plant . . 6*2 8-9 
■< experimental plant 6*2 6-9 
binervosa zone of a lateral . . 4*8 6*9 
(one year in greenhouse) 
crest of a lateral 4*1 4-1 
\ (one year in greenhouse) 
Tall Form. S. binervosa , Main bank plant . . . 5*5 5*5 
The glands, speaking generally, are somewhat more numerous on 
the under than on the upper side of the leaf. It is interesting to note 
that in the case of the various forms of ther dwarf binervosa , in which 
the amount of water available for absorption would be expected to be 
1 Solereder, Fig. 113, d-f, p. 497. 
2 Or possibly by the elimination of the stalk cells, and the modification of neighbouring cells to 
form the subsidiary cells. 
