(43) 



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 



