240 Salisbttry . — The Extra-floral Nectaries of the 
Plumbagineae, and Frankeniaceae, all of which secrete osmotically active 
substances. 
The secretory activity of the nectaries appears to be due to osmotic 
action, largely independent of root pressure. The thin-walled character of the 
basal cells may be associated not only with the need for ready permeability, 
but also with communication of the turgor pressure of the surrounding tissue. 
The experiments seem to show that the plant can accommodate itself 
to local inequalities of transpiration, and that these nectaries are, from the 
physiological point of view, water-secreting organs whose action is dependent 
upon osmosis. 
To summarize the reasons for this view: 
(1) The structure of the glandular trichomes agrees with that of the 
leaf-glands. 
(2) The nectaries function at an early stage in the development of the 
organs upon which they are borne, and therefore at a period when excess 
of turgidity would be most injurious. 
(3) There is a marked increase of secreted fluid accompanying an 
increase of humidity. 
(4) The nectaries stand in close relation to the vascular bundles, and 
there is an increase in their number and extent accompanying an increase 
in complexity of the vascular supply. In those species which have a 
reduced petiolar vascular supply a reduced or simplified nectary is 
present. 
(5) In no case were glands observed to be visited by ants, either 
in the exotic species at Kew, or in P. Convolvulus in its native haunts. In 
this latter some dozens of plants in various situations were kept under 
observation, and in no instance were they found to be visited by insects. 
That some physiological purpose was served by extra-floral nectaries 
was suggested by Schimper (14). That this was, in the case of floral 
nectaries, of a hydathodal nature was advocated by Burck (4), and the same 
view in respect to floral nectaries has been put forward by Renner (12) and 
Schwendt (15, p. 42). The ant protection theory, whilst it is undoubtedly 
applicable in certain instances, has only been proved for a few. Aufrecht (2) 
and others have remarked on the absence of insects during their observations. 
Such protection must be regarded as an altogether secondary adaptation. 
An admirable historical summary of the views regarding the function 
of extra-floral nectaries is given by Niewenhuis-Uxkull (11). 
Bonnier (3) showed for floral nectaries that the maximum secretion 
took place when the soil and air were saturated. 
Ono (12), who worked on numerous extra-floral nectaries including 
those of P. sachalinense and P. cuspidatum , summarized his results as 
follows. ‘ Among different external circumstances, moisture seems to be of 
the greatest importance ’ (p. 18). 
