Chapter X: Cryptogams (3)

=419. Explanation of the diagram.=—An attempt to show the general course of plant evolution up to the present time is made in the accompanying diagram. The four great divisions, Thallophytes, Bryophytes, Pteridophytes, and Spermatophytes, are represented by spaces between four horizontal lines arranged one above the other in the order of their succession in time and complexity of organization. It should be borne in mind that these dividing lines are not sharply defined in nature, but overlap or indent the territory between them with varying degrees of irregularity, like the coast line on a map. The relative positions of the different orders we have been considering are represented by upright and diagonal lines, the general course of which, as indicated by the arrows, is intended to give an idea of the trend of evolutionary progress in the particular group represented by each line. No one of these lines is made to originate directly in any other, because, with the possible exception of the monocotyls, we have no authority for asserting that any such direct connection exists between plants as we know them, but only that certain types give evidence of descent from a common ancestry. This lack of certainty is expressed by placing the point of origin for any given line in more or less close proximity to the one which is supposed to be the nearest living representative of the common ancestor. The line of ferns, for instance, is depicted as originating in the region of the bryophytes, somewhere in the neighborhood of the liverworts, but the two lines nowhere come in contact, because there is no evidence that any fern, living or fossil, is directly descended from any particular kind of liverwort known to us. With these explanations, the diagram shows, in a rough way, the generally accepted view of plant relationships as based on the evidence at present before us. But in questions of this sort it is wise to keep in mind the blunt remark of a famous old American statesman, that “only fools and dead people never change their opinions.”

Field Work

1. If you live in the country, study the appearance of plants
affected with blights, smuts, rusts, and mildews, and learn to
recognize the different kinds of disease by their signs. Notice
which kinds are most prevalent in your neighborhood, and what
plants are most affected by them.

2. Notice the different kinds of mushrooms you find growing wild.
Observe the difference between those that grow on the ground and
those that grow on logs, stumps, and trees; between those found
in the woods and those in open ground. Find out how those on the
ground get their nourishment. Uncover the mycelium, and notice
the extent of its surface. Examine the soil and find out if it
contains anything upon which they could feed. Note the prevalence
of shelf fungi on trees. Examine the condition of the wood where
they grow, and decide in what ways they injure their hosts. Notice
whether they abound most on healthy or on decaying trunks and
boughs, and decide whether this is because the fungus prefers that
kind of host, or whether the injury it does causes the decay, or
whether both causes operate together. Notice what fungi grow on
different trees, and study their preferences in this respect.

3. Observe the different kinds of lichens found in your walks
and try to distinguish the three classes. Which kind are most
abundant in your neighborhood? Which least so? Note the situations
in which you find each kind growing, whether on stumps, trees,
rocks, or the ground. Consider how the algæ and fungi aid each
other in the different positions; could either, for instance, exist
independently on bald rocks? Notice on what kind of trees the
different lichens seem to thrive best and on which poorly or not at
all, and whether the character of the bark—rough, smooth, scaly—has
anything to do with their choice of a habitat.