Minnesota Plant Life. 515 



Having" been liberated either from the stalk to which they 

 were attached or from the chamber in which they were confined, 

 spores become units for which various distributional devices 

 have arisen. They are borne away by gusts of wind, by cur- 

 rents of water, or by insects or birds that have picked them 

 up either by accident or design. Sometimes special assistance 

 is given them by the parent plant — directly, as when a liver- 

 wort produces, in its capsule, writhing cells or clatcrs, by the 

 struggling- movements of which the spores are scattered; in- 

 directly, as when the flowering plant secretes honey near where 

 its pollen spores are formed, thus attracting insects that may 

 act as carriers. A great many structures in the plant world 

 have arisen to assist in the distribution of spores. Among 

 these may be mentioned the erect stems of moss capsules, serv- 

 ing to lift the spores into the wind, the teeth at the margin of a 

 moss capsule, serving to sift out the spores at favorable times, 

 the spring-back of the catapult-like spore-case of true ferns, and 

 the explosive pollen-sacs of some flowering plants ; but no 

 adequate idea of the variety that exists can be given in a limited 

 space. 



Sometimes spores attend to their own distribution by the 

 development of motile organs, such as swimming threads, as 

 in fish-moulds or green felts, cr by the production of gas bags 

 by which they float more easily in the wind — a condition that 

 exists among the spores of the pines. Many pollen-spores have 

 viscid or spinous surfaces that facilitate their attachment to 

 the bodies of insects, and in general, the surface of the spore 

 is commonly adapted to aid in distribution. 



Not infrequently spores are distributed en masse rather than 

 separately. Orchids and milkweeds furnish good illustrations 

 of this when several hundred of their spores cohere in little 

 club-shaped or saddlebag-shaped clusters and these are carried 

 away as units upon the bills or legs of insects. Similar aggre- 

 gates of spores are produced in many lower plants, as in the 

 water-ferns, AzoUa and Salvinia; and a somewhat related con- 

 dition characterizes the green felts. In the latter each motile 

 perfect reproductive body is covered with swimming lashes, 

 indicating that it is equivalent to an undivided group of spores, 

 each one of which, if separate, would have had its own pair 

 of lashes. 



