MODES OF REPRODUCTIOISr IN PLANTS. 



161 



rows, and the sporangia are raised on long stalks. Fig, 1 shows at ^ a 

 portion of Marchantia polymorphia, with an upright receptacle, h, 

 hearing the male organs. In Fig. 2 is seen a stem of Plagiocliila 

 asplenioides ; a is a ripe sporangium, and b one that has opened. 



In the mosses, the next class in the upward scale of plant-life, the 

 spore germinates by producing a fine green thread, which branches and 

 forms a plant much resembling many of the filamentous fresh-water 

 algae. In the order Sphagnacem this " alga form " is a flat expansion 

 similar to the sexual generation of many liverworts. Fig. 3 shows 



Fig. 3. 



the germinating spores of Thmaria hygrometrica, at A, and the fine 

 branching, green threads (B) that are afterward produced. The true 

 moss-plant, with its small stems, and fine, regularly arranged leaves, 

 originates from specialized cells in the protonema, or alga form. K, in 

 Fig. 3, shows the rudiment of a leaf -bearing axis. On the conspicuous 

 moss-plant, arising from such small beginnings, the sexual organs are 

 borne. They are usually produced in clusters at the ends of the leafy 

 axes. Most mosses have only one sex represented in a single tip, and 

 some species have the separation of the sexes so complete that a plant 

 bears only one kind. Mosses, as well as trees and shrubs, are some- 

 times monoecious or dioecious. Fig. 4 shows the male and female 

 organs very much magnified. The antheridium. A, is a stalked, club- 

 shaped structure, inclosing a large number of sperm-cells, b, each of 

 which produces a spiral spermatozoid, c. These minute bodies move 

 rapidly by means of two cilia, and find their way to the neck of the 

 female organ, B. The germ-cell, b, to be fertilized, is at the base of 

 the long structure, with a mucilaginous channel, h, leading down to 

 it. After the spermatozoids have united with the germ-cell, the latter 

 soon begins a new growth, and a young sporangium results. Fig. 5 

 shows different stages in the development of the sperm-case ; and in 



VOL. XXV. — 11 



