Minnesota Plant Life. 37 



branches which have fallen into the water. Their characters 

 are all more or less similar to those which have been already de- 

 scribed. 



Leaf-dwelling algae. One remarkable alga, very rare in Min- 

 nesota, is found in a peculiar habitat. On the leaves of the 

 jack-in-the-pulpit there may occasionally be noticed watery blis- 

 ters, in which, under the epidermis of the leaf, slender green 

 threads branch and grow, giving a pale green tint to the central 

 portion of the blister. Here is an example of a parasitic alga, 

 forms similar to which are more frequent upon leaves in tropical 

 forests than in temperate regions. 



Sphere-algae. Another bright green alga which appears to 

 be uncommon in Minnesota, but sometimes forms floating tufts 

 of slender green threads in the waters of overflowed meadows, 

 is remarkable for its production of true eggs and spermatozoids. 

 As in the pond-scum, each thread of the body is an unbranched 

 row of joints or cells all of which are shaped like long glass 

 cylinders closed at each end. In some of these cells the living 

 contents break up into a dozen or more spherical green eggs, 

 which lie close against the wall of the mother-cell and, by fer- 

 ments which they secrete, make little punctures. Other cells 

 of the filament convert their contents into thousands of motile 

 spermatozoids which dart about in the cell-cavity in a compli- 

 cated dance which finally results in some of them perforating 

 the wall, and through the apertures all escape into the water. 

 After swimming about for a time many of them find their way 

 through the pin-holes which the eggs had made in the walls of 

 their mother-cells. They enter the egg-mother-cells and one of 

 them buries itself in the substance of each egg. When the eggs 

 in a tube have been thus fecundated each encloses itself in a 

 spiny membrane, assumes an orange color, and after the wall 

 of the mother cell has broken or dissolved each fused-body 

 escapes into the water and divides internally into a little group 

 of spores. The spores in turn escape and develop new filaments 

 of the alga. This alga is called the sphere-alga on account of 

 the tubes full of spherical eggs which characterize it. 



Green felts. Among the other algae of this group should 

 be mentioned the green-felts which form plush-like masses in 

 springs and ditches. They are remarkable for the unjointed 



