54 



Minnesota Plant Life. 



some blackberries are three-celled instead of two-celled, and the 

 cluster-cup masses on some gooseberries are developed upon 

 protuberant filament-aggregates. Perhaps the three most re- 

 markable forms are the pine-knot fungi which form knots some- 

 times as large as bushel baskets upon the branches of pine trees, 

 the so-called "cedar-apples," which occur as curious bunches, 

 the size of one's thumb and armed with orange horns, on the 

 junipers, and the witch's brooms on balsam trees. These latter 

 are immense, disordered tangles of branchlets forming masses 

 sometimes several feet in diameter. The disordered branching 

 is caused by the growth of a rust-fungus of the cluster-cup sort 

 in the substance of the twigs. When this fungus fruits it pro- 

 duces its reproductive structures upon the leaves of the balsams, 

 where they recall strikingly the rust produced on barberry 

 leaves. The witch's broom is a notable object in the swamps 

 of Minnesota. When large ones are developed on the balsam 

 trees they look like great crows' nests up in the branches and 

 very often birds and animals use the thick tangle of twigs to 

 conceal their own dwelling places. On the spruce trees there 

 is a similar tangle of branches produced by the agency of insects, 

 but there is of course no development of the characteristic clus- 

 ter-cup fruit-bodies upon the leaves. The related pine-knot 

 fungus does not commonly fruit every year, but sometimes the 

 whole surface of the knot will be found covered with the little 

 orange pustules of the rust. 



