110 



THE AMERICAN BOTANIST 



from the lichen it thrives by itself. The fungus, on the con- 

 trary, must have some kind of manufactured food and appears 

 to favor taking it from the alga. If a fungus spore begins 

 germination in the vicinity of an alga colony, it soon captures 

 some of the independent plants and a lichen results. Artificial 

 lichens of this kind have actually been made in the laboratory 

 and grown to maturity. When the lichen fruits it is the fun- 

 gus and not the alga that is reproduced, though in some groups 

 it is the lichen itself that seems to multiply by producing min- 

 iature lichens consisting of a few alga cells held together by a 

 strand of the fungus. The group of fungi to which those of 

 lichens are manifestly related is the Ascomycetes or sac fungi. 

 The black goblet {Urmila cratcriuin) and the lilac mildew 

 (Micro'Sphaeria) are examples of common non-lichen forms 

 belonging to this group. 



Plant Successions. — We are accustomed to think of the 

 vegetation of any particular tract as fixed and settled, but this 

 is far from the truth. After the retreat of the great ice sheet 

 which once covered the greater part of the Northern States, 

 the land was without vegetation of any kind, and it was only 

 little by little that plants came in to occupy it. In most cases 

 there was a regular succession of plant forms, each new group 

 displacing the previous one. This succession was probably dif- 

 ferent according as the land was sand, clay, or rock. In Coul- 

 ter's ''Plant Breeding," the succession that led up to the white 

 pine forests on the sandy soil of northern Michigan is outlined 

 as follows : ''At first there were only a few scattered annual 

 plants which dying, added a small amount of humus to the 

 sand. When sufiicient humus had accumulated the heath for- 

 mation came in. The plants of this formation are the huckle- 

 berries and allied plants. During this period the ground was 

 so completely covered that competition of plant with plant be- 

 gan. The continued accumulation of humus finally made the 

 pine forest possible and young specimens of this tree began to 



