334 PRACTICAL COURSE IN BOTANY 
Practical Questions 
1. Have lichens any economic value? (384.) 
2. In what way are they most useful? (820.) 
3. Do you find them, as a general thing, on healthy young trees and 
boughs, or on old ones, and those showing signs of decay ? 
4, Do you ever find them growing on trees or other objects in densely 
inhabited areas, — cities, large towns, and manufacturing centers? 
5. Do they grow more thickly on the shady (northern) side of rocks, 
walls, and trees growing in the open, than on the sunny and (presumably) 
warmer sides ? 
6. Mention some ways in which a growth of lichens might be beneficial 
to a tree. 
7. In what ways could it be harmful? 
V. LIVERWORTS 
Mareriau. — Liverworts can generally be found growing with mosses 
in damp, shady places, and are easily recognized by their flat, spreading 
habit, which gives them the appearance of'green lichens. Marchantia 
polymorpha (Fig. 475), one of the largest and best specimens for study, 
is common in shady, damp ground throughout the states. It is dicecious, 
and specimens bearing both male and female organs should be provided. 
Lunularia, a smaller species that can be recognized by the little crescent- 
shaped receptacles on some of the divisions of the thallus, is abundant 
in greenhouses on the floor, or on the sides of pots and boxes kept in damp 
places; but the spore-bearing receptacles are seldom or never present, 
the species being an introduced one and possibly rendered sterile by 
changed conditions. Anthoceros (Fig. 426) and leafy liverworts, such 
as that shown in Fig. 484, also make good examples for study. 
EXPERIMENT 97. WHY ARE THE UPPER AND UNDER SIDES OF A LIVER- 
WORT DIFFERENT? — Plant a growing branch of marchantia, or of any 
flat, spreading liverwort, in moist earth so that the upper side will lie next 
the soil, and watch for a week or two, noting the changes that take place. 
What would you infer from these as to the cause of any differences that 
may have been observed between the two surfaces? 
385. Examination of a typical liverwort — The thallus. — 
The broad, flat, branching organ that forms the body of the 
plant is the thallus. Examine the end of each branch; 
what do you find there? Are the two forks into which the 
apex of the branches divides equal or unequal? Compare 
the growing end with the distal one; does it proceed from 
