(221) 
City and threatens the entire destruction of this valuable 
tree. Some plants grow above the surface of the ground, 
as in the case of the morel; while others are subterranean, 
as in the case of truffles. In case 32 has been installed 
specimens and illustrations of crown-gall, an abnormal 
growth which is caused by minute plants known as bacteria. 
This peculiar growth is commonly known as vegetable 
cancer on account of its close resemblance to the cancer 
of the human body. The disease is very destructive to 
trees and shrubs of various kinds. Next in order are the 
alga-like fungi; these vary in form from simple masses of 
protoplasm to simple or branching threads. Here belong 
many of the moulds and similar forms which grow both on 
other plants and on animals. The fifth and in many re- 
spects the most interesting of all the groups is that con- 
sisting of the lichens (cases 33 to 36). The lichens have 
commonly been considered to form an independent sym- 
biotic group, each lichen being supposed to consist of a 
fungus and an alga living together, the one nourishing the 
other, but, according to some of the more recent students 
of the group, the lichens are simply fungi that live parasiti- 
cally upon algae. The lichens are quite familiar to most 
people as plants of more or less leathery texture growing on 
rocks, on poor soil or on the trunks of trees. 
A step forward brings us to the Bryophyta. These are 
seedless green plants, most of which possess roots, stems, and 
leaves, but have no vascular tissue (cases 37 to 48). This 
group is best known through the mosses, which form its 
largest division. Of somewhat simpler structure are the 
hepatics or scale-mosses (cases 37 to 40). The stems and 
leaves of the hepatic plant are sometimes combined into 
a flat thallus-like body which creeps closely on the ground or 
other objects and resembles in aspect some of the lichens. 
The leaves, when present, are usually more delicate in 
texture than in the true mosses and they do not have a 
midvein. These differences alone enable one to distin- 
guish a hepatic from its relatives by the unaided eye or at 
