766 PHYSIOLOGY. 



here commence with the Thallogens, and proceed upwards to 

 those plants of a more complicated nature, instead of alluding 

 to them, as we then did, in the inverse order. 



A. Reproduction of Thallogens. — The sexuality of all Thallo- 

 gens has not been absolutely proved, but only concluded from 

 analogy. It is in the Algae that the sexes have been most clearly 

 shown to exist. The reproductive organs of Fungi and Lichens 

 have been already described, and but little is positively known 

 as to the mode in which they are reproduced. Oersted, indeed, 

 has recently described the impregnation of oogonia on the 

 mycelium of Agariciis, and De Bary has seen what he thinks 

 may be Antheridia in some of the Ascomycetes ; but, in the want 

 of definite details, it will be only necessary for us to give a sum- 

 mary of the modes of reproduction in the Algae or Sea-weeds. 



Reproduction of Algoe. — The propagation of Algae takes place 

 in two very distinct ways ; namely, by conjugation, and by the 

 direct impregnation of naked spores or germ corjyuscles hy ciliated 

 sipermatozoi'ds. Each mode is also subject to modifications. 

 We can only briefly allude to the subject here. The existence 

 of sexes has been proved in members of all the sub-orders 

 of Algae, and the actual impregnation of the female corpuscle 

 has been directly observed. 



1. Conjugation. — This process has been noticed in the Diato- 

 maceae generally, and in certain Chlorosporeae, It consists in 

 the union of the contents {endochrome) of the cells of two fila- 

 ments (fronds) {fig. 831), and the formation of a germinating 

 spore by their mutual action. No diiference can be detected in 

 the structure of the conjugating cells. 



Two methods of conjugation may be noticed — one, which occurs 

 generally in the Diatomaceae and Desmidiaceae ; and the other in 

 certain of the Chlorosporeae. In the first mode {figs. 1097 and 

 1109), two individuals, each of which is composed of a single 

 cell, approach one another, the external cellulose membranes 

 bounding their respective cells burst, and the contents of the 

 two, invested by a primordial utricle, issue from the orifices thus 

 produced {fig. 1109), intermingle in the intervening space, and 

 form ultimately, by their mutual action, a rounded body {fig. 

 1097), called a resting or inactive spore, which ultimately ger- 

 minates. The contents of the spore are green and granular at 

 first, but ultimately become brown, yellow, or reddish. These 

 resting spores, which are furnished with two coats {fig. 1097), 

 are sometimes called sporangia, because they ultimately produce 

 two or more germs in their interior, and are not therefore simple 

 spores. 



In the other mode of conjugation, which occurs in certain 

 Chlorosporeae, as in Zygnema {fig. 831), the cells of two fila- 

 ments develope on their adjoining sides a smalltubular process; 

 these ultimately meet and adhere, and the intervening septum 



