Yapp . — Fruit-dispersal in Adenostemma viscosum. 315 
part of the style forms a cylinder of thickened cells. Separation in this 
case occurs by the breaking down of the thin-walled tissue at the base of 
the style (Fig. 10). 
The fruits themselves are separated from the torus in a manner 
essentially similar to that described for the abscission of the corolla and 
style. Each fruit is attached to the base of a shallow pit in the torus, and 
is detached by the breaking down of fragile parenchymatous cells lying 
between the thick-walled epidermis of the lower part of the fruit, and 
a funnel-shaped layer of thickened cells of the torus (Fig. 11). Only those 
cells which form the rim of this ‘ funnel ’ are epidermal, the remaining cells 
being of a more deeply seated origin 1 . 
The vascular strand which supplies the ovary contains but few vessels, 
but is accompanied by numbers of short sclerotic cells (Fig. 11), which 
probably help to render the strand itself more brittle. In any case, the 
strand breaks across with great readiness. 
In all these cases, the actual thickening of the cell-walls (including 
those of the sclerotic cells mentioned above) takes place only during the 
maturation of the fruit. 
As we have seen, the process of fruit-dispersal in Adenostemma is 
facilitated by the abscission of certain parts of the flower, and later, of the 
fruit itself : and moreover, that this is aided by the topographical relations 
of hard and soft tissues. But such relations really do little more than 
determine the direction of rupture. The actual breaking down of the 
fragile tissues is probably due partly to their desiccation as the fruit ripens, 
and partly to the mechanical strains set up by such causes as the increasing 
convexity of the torus (Figs. 1-3). 
The term ‘ absciss-layer,’ which has sometimes been used in speaking 
of corollas 2 , would appear to be, in cases such as that described above, 
somewhat of a misnomer. In those instances where (as in many Onagra- 
ceae) a definite meristematic layer is formed in connexion with the cutting 
off of parts of the flower, the term might be retained. But where separation 
of an organ depends more on the relative disposition of hard and soft 
tissues than on meristematic activity, some such term as c absciss-mechanism ’ 
would seem preferable. 
Although, as stated at the commencement of this paper, the genus 
Adenostemma appears to be unique in possessing a glandular pappus, yet 
certain other Compositae have glandular structures which aid, more or less 
effectively, the process of fruit-dispersal. Of these, the genus Siegesbeckia , 
which has glandular hairs on the involucral bracts, appears to be the most 
1 Not (at least in Adenostemma ) merely epidermal, as stated by Kraus, Ueber den Bau trockner 
Pericarpien. Pringsheim’s Jahrb. f. wiss. Bot., 1866, vol. v, pp. 123-4. 
3 Von Mohl, Ueber den Ablbsungsprocess saftiger Pflanzenorgane, Bot. Zeitung, i860, vol. xviii, 
p. 276, speaks of certain corollas as possessing an absciss-layer (Trennungsschicht), but he nowhere 
describes its structure. 
Y3 
