144 ANGIOSPERMAE—DICOTYLEDONES 



one, which is usually yellowish-white with ill-developed nectar-guides. The upper 

 petals of the latter form are rarely bluish or violet (of. p. 143). 



Miiller says that the blossoms of the small-flowered variety — which are protected 

 against the ravages of insects — wither after two or three days, by which time the 

 fruits have been set. The flowers of the large-fiowered variety, on the other hand, 

 do not wither for two or three weeks, and then usually without having set fruits 

 (cf. p. 142). 



MacLeod found on sand-dunes in Belgium — at Blankenberge — a variety 

 possessing remarkably large flowers, with black spots on the under-side of the 

 stigma. The stigmatic papillae of this variety are more numerous than in flowers 

 from other habitats. 



Wittrock (' Viola-Studier ') says that the two constituent parts of the nectar- 

 guide on the odd petal, i. e. the ' nectar-spots ' at its base, and the ' nectar-streaks ' 

 which radiate forwards from it, are constant in colour in the same variety of 

 the pansy, even though the colour of the petals varies. The nectar-spots are 

 yellow or orange, and the honey-streaks dark violet. The spur is always violet. 



The hair-fringed groove on the lower petal serves — -according to Wittrock-^ 

 as a ' pollen-magazine,' in which the falling pollen is collected and stored till an 

 insect visits the flower. The hairs possess knot-like thickenings, which appears 

 to be an adaptation for holding the pollen-grains fast. The front part of the groove 

 forms a ' pollen cavity ' open above, while its posterior narrower part constitutes 

 a ' pollen-canal.' The pollen falls into the pollen cavity through an opening between 

 the membranous appendages of the two lowest stamens, which is situated exactly 

 above it. 



Wittrock has also demonstrated by direct experiment that the hairs on the 

 bases of the limbs of the lateral petals serve to protect the reproductive organs 

 and the pollen magazine against rain. They are also — as earlier investigators sup- 

 posed — of use as footholds for pollinating insects. 



The petals are much smaller and relatively much broader in the younger flowers 

 than in the older ones. 



The flowers of the plant differ according to the season: in spring and early summer 

 the petals are much larger, broader, and more brightly coloured than in late summer. 

 The nectar-streaks are often absent in late summer and autumn, but the nectar-spots 

 and nectaries are constant. 



It is exceptional to find flowers of very different colour on the same plant. 

 In one such case that was accurately investigated this peculiarity was limited to 

 the hottest part of the summer, when the colour varied from violet to white. In 

 spring and autumn, on the contrary, the plant produced only violet flowers. Reasoning 

 from this and other cases, Wittrock thinks it probable that a high temperature has 

 an injurious eff'ect upon flower production in the varieties of the pansy, causing 

 smaller and paler blossoms to be developed. 



For the first two or three days of anthesis — which lasts about a week — the 

 petals of V. tricolor are nyctitropous. The two upper petals bend forwards in 

 the evening, assuming an almost horizontal position, the lateral petals curve a little 

 inwards, and the lowest petal becomes slightly concave by the upward bending 

 of its edges. Besides these movements — which were overlooked by earlier investi- 



