150 
POLLEN PRESENTATION IN 
disc-florets are all perfect and all similar in structure. The tips of the five 
corolla lobes are thickened slightly and black in colour on their outer surfaces ; 
within they are dull green. The disc-florets mature centripetally, and a well- 
grown inflorescence, in suitable weather, opens on three successive days. 
The inflorescence opens about 8.30 a.m., on a bright, warm day. 
Throughout the open period there is a well-marked heliotropic curvature 
of the peduncle, maintaining the surface of the inflorescence fully exposed 
to the sun’s rays. If the inflorescence be a young one, open for the 'first time, 
the florets of the outermost ring open, exposing the tips of the staminal tubes, 
which can be seen projecting slightly (Fig. 1). 
Then the process of pollen presentation commences. The tips of the anthers 
form five triangular flaps, which at first completely close the tube (Fig. 1). 
These are forced apart by the rapidly expanding style. 
The style has a cylindrical, thickened, head, covered with minute spines 
formed by the conical epidermal cells with which it is covered (Fig. 5). These 
cells have a dark blue cell sap, rendering the style head almost black in colour. 
Round the base of the head these epidermal spines are comparatively long, 
forming a brush which effectively clears the spiny pollen grains from the 
staminal tube, when the style elongates (Fig. 2). 
During pollen presentation, it is- only the slender, stalk-like portion of 
the style that elongates, thrusting the head, thickly covered with yellow 
pollen, far out of the staminal tube. 
The process of pollen presentation is remarkable for its rapidity, the com- 
plete elongation of the style occupying about five minutes under favourable 
conditions. On comparing young and completely extended styles, it is seen 
that the elongation is due to the rapid distention and elongation of the cells 
of the stalk portion of the style, no doubt due to the formation in them of 
osmotically active substances, followed by rapid absorption of water (Figs. 5 
and 4). 
The fully extended style is sensitive to contact stimulus, and will curve 
in the direction from which the stimulus comes (Fig. 3). 
The cause of the movement is probably the sudden alteration in the 
permeability of the protoplasts of the style stalk, and a consequent loss of 
turgidity of the cells, owing to the excretion of water into the intercellular 
spaces with which the style is well provided. In order to produce curvature, 
the loss in turgidity must take place to a greater extent on one side of the 
stalk than upon the other. 
Such a response is of considerable value in cross pollination, as is at once 
seen if a bee is observed during its visit to an inflorescence. The bee most 
frequently alights on the ray-florets, and walks to the centre of the inflor- 
escence, turning round several times as it investigates the expanded disc- 
florets. 
