34 



THE POPULAR SCIENCE MONTHLY. 



wall of the mother-cell, thus leaving the grains separate in the 

 anther. In entomophilous plants, on the contrary, the pollen is 

 often viscid, or else studded with points, so that it may in some 

 way adhere to the legs, bodies, or probosces of insect visitors. 



While, as a rule, the pollen grains are free in the anther cells, 

 there are two families, those of the Orchids and the Milkweeds, 

 in which the grains are developed in a peculiar manner. In these 

 families, but especially in the former, the grains cohere to one 

 another by means of a viscid matter, and thus form one mass, 

 technically known as a pollinium (Fig. 6). The cohesion is 

 brought about by the walls of the mother-cells remaining, and 

 so binding the grains more or less completely together. The co- 

 hesion is but slight, for, on the application of a pol- 

 linium to the viscid surface of a stigma, several 

 grains are left on it, and thus one pollen mass will 

 serve to fertilize several flowers. One of the most 

 striking features of these pollinia is that the stem 

 or caudicle undergoes on exposure to the air an act 

 of depression (Fig. 7), so that while it stands erect 

 on its first withdrawal from the anther cell, in a 

 short time a contraction in the substance of the 

 caudicle is observed, and then the pollinium be- 

 Fig. 6,-Pollinium comes horizontal. 



While in ordinary cases pollen is yellow, there 

 are instances in Avhich this feature varies. For example, in one 

 form of flower of loose-strife {Lythrum scdicaria) it is green. In 

 the willow-herb (Epilobium angustifolium) it is blue. In the tulip 

 it is black, and in mullein ( Verbascum) it is red. 



The seeds of flowering plants are produced by the action of the 

 pollen on the stigma. Though both bulb and seed bear in them- 

 selves the potentiality of the future plant, the two are very differ- 

 ent. This difference can be well stated by saying that the bulb 

 perpetuates the individual, and the seed the species. When the 

 anther is nearly ripe, its inner wall be- 

 comes thinner and thinner, either along 

 certain lines or in particular spots. The 

 process continues until by pressure the 

 wall is ruptured at these places and the 

 pollen escapes. When the grains are 

 placed upon the viscid stigma, the moisture absorbed by endos- 

 mose causes the contents to swell, and the intine bursts through 

 the thin places in the extine and protrudes in the form of a tube, 

 which penetrates the stigma to the ovary (Fig. 8). While the 

 tube is at first only a projection of the intine, it afterward becomes 

 a growth, for it is many times larger than could be contained in 

 the grain. It is, therefore, nourished by the conducting tissue of 



Fig. 7. Pollinium after De- 

 pression. 



