other stigmas and the entrance to the nectary. The pollinia, which lie in the anther cells, are club-shaped, the head of the club consisting of a number of packets of pollen grains united by thin elastic threads: the stalk of the club ends in a disc with a ball of very viscid matter on its under side, lying in the rostellum. The anther cells open when ripe, exposing the pollinia; the rostellum is very delicate, and is ruptured by the slightest touch, exposing the viscid balls.

The problem is to transfer the pollinia from one spike of flowers to another. The manner in which this problem is solved through the agency of insects is as follows.

The insect, alighting on the labellum, pushes its head into the flower in order to reach the spur with its proboscis. In doing this it knocks against the rostellum, displacing its covering membrane and exposing the viscid balls to which the pollen masses are attached. On withdrawing its head the pollen masses come away firmly cemented to it and standing erect. In about thirty seconds the viscid disc contracts, causing the pollen mass to bend forwards through an angle of 90°, so as to become horizontal. By this contraction the pollen mass will be in a position to be applied directly to the stigma, when the insect visits the next flower. This manœuvre can be imitated by pushing the point of a pencil into a flower as shown in the figure, when the pollen masses will come away fixed to the pencil. (Fig. 27.)

In this way an insect flying from flower to flower

K

effects cross-fertilisation regularly, and humble bees were actually watched in the act of fertilising by Hermann Müller. He saw them insert their heads into the flower and emerge with the pollinia attached, visit other flowers on the same spike, where they tried, more or less ineffectually, to rub off the pollinia, and finally fly off to other plants. Out of 97 humble bees which he caught, 32 bore the pollen masses of orchids. He proved that the bees visit the flower to obtain the fluid in the nectary, the walls of which they pierce with their maxillæ. Moreover, he timed the bees and found that they spent three or four seconds at each flower; two or three seconds being sufficient to fix the pollinia. The average time spent at a given spike of flowers was twenty to twenty-two seconds, the bees then flying to another spike. In twenty-five to thirty seconds the pollinia were depressed and cross-fertilisation ensured.

The beauty and odour of flowers and the storage of honey are thus due to the existence of insects, and in a large number of cases the actual insects are known which effect cross fertilisation. Such is the case with regard to all conspicuous flowers: honey is secreted in order to attract insects, and the flowers are large and conspicuously coloured, so as to be readily seen by them. A striking illustration of this is seen in the common Clover. Darwin showed that by protecting 100 flowers with a net, not a single seed was produced from them; whereas the 100 flowers which were outside the net were visited by bees and produced 2720 seeds. Hence, but for humble bees,

which are the only insects visiting the common red clover, there would soon be no clover.

Large conspicuous flowers are visited much more frequently and by many more kinds of insects than are small inconspicuous ones. The long tubular corolla of many flowers is acquired so that certain insects alone should be able to get at the honey, these insects being the ones best suited for fertilising the flower.

The bright colour of the whole flower is to attract insects at a distance; the coloured dots and lines on the petals serve to guide it to the store of honey. This fact was proved by Darwin, who cut off the petals of Lobelia, and found that these flowers were then neglected by bees, which were perpetually visiting the other flowers.

In the case of flowers which are fertilised by means of the wind, such as grasses and trees, the flowers are small and not gaily coloured, and possess an enormous amount of pollen and a very large stigma. Moreover, in localities where insects are few in number, we find the flowers very insignificant in colour: for example, in the Galapagos Islands, which have only one butterfly and no bees.

White flowers are fertilised by nocturnal insects, chiefly moths. These flowers are always odorous, the jasmine and clematis for example, and often odorous only at night. Alpine flowers, again, are peculiarly beautiful, and the size of individual flowers is increased owing to the comparative scarcity of insects in the places where they grow, and the consequent necessity of attracting them from afar.

FRUITS.

Fruits consist of seeds with surrounding envelopes of various kinds, and require to disperse their seeds so as to reach places favourable for growth and germination. Dispersion of the seeds is effected in some cases, such as the dandelion, by means of the wind; in the edible fruits, on the other hand, it is effected by the fruit being swallowed by animals as food. Fruits are divided into two great groupsattractive fruits and protective fruits.

ATTRACTIVE FRUITS are soft, pulpy, and agreeable to the taste-such as the cherry, grape, strawberry, &c. and are devoured by birds or mammals. In these the seeds themselves are hard, and pass through the animal unchanged. It is probable that every brightly-coloured pulpy fruit serves as food for some species of bird or mammal.

PROTECTIVE FRUITS, such as nuts. In these the part that would be eaten by an animal is the seed itself, and this is protectively coloured, being green while on the tree, and turning brown as it ripens and falls to the ground. Many seeds are specially protected, such as the chestnut by its prickly coat, and the walnut by its nauseous covering.

It thus appears that we owe the existence of many flowers to insects, and of many fruits to birds and mammals. This is an excellent example of the interest imparted to everyday life by the theory of Natural Selection, which tells us that we have merely to watch closely, to note carefully what is going on every day before our eyes, in order to obtain the