32 



THE AMEEICAN MONTHLY 



[February, 



time in the life-history of even the 

 simplest plants; for only by the union 

 of opposite elements can the vegeta- 

 tive life be maintained. We can hard- 

 ly suppose that a single cell could give 

 rise to an unlimited progeny by divi- 

 sion continued indefinitely. The vi- 

 tality of the later generations would 

 finally be exhausted, and the species 

 could then only be maintained by 

 ■ some kind of sexual union which re- 

 news and vivifies. But while we say 

 this, it should be understood that the 

 characteristics of the male and female 

 elements of plants and animals are 

 not known. Conjugation signifies the 

 union of these two elements, but in 

 the lowest stages of life no difference 

 can be distinguished between them. 

 It is even doubtful if they possess 

 any distinguishing characters, for 

 when any two particles of protoplasm 

 come together and coalesce, we call 

 it true conjugation; and the living 

 contents of a single cell may separate 

 into two parts, one part passing to 

 one end and the rest to the other end 

 of the cell, after which the two por- 

 tions may again unite and form a 

 spore. 



Passing a step higher in the scale, 

 we find many plants that conjugate 

 by the union of the cells of two fila- 

 ments and the intermingling of their 

 protoplasm results in the forma- 

 tion of a spore, from which 

 new plants develop. This is the 

 case with all the family Zygnemaceae, 

 the fruit of which is called a 

 zygospore. As an example of this 

 interesting process, we may choose a 

 species of Spit'ogyra, a filamentous 

 alga common in our roadside ditches, 

 which has received its name from the 

 spiral arrangement of the coloring 

 matter within the cells. When the 

 time of conjugation arrives, the cell- 

 contents lose their regular arrange- 

 ment, and finally two filaments, lying 

 side by side, will send out short ex- 

 tensions which gradually approach 

 each other, unite by their ends, after 

 which, by the dissolution of the ter- 

 minal partition, there will soon be 



formed tubular passages from the cells 

 of one filament to those of the other, 

 and the two plants will be united as 

 by the rounds of a ladder. Then all 

 the protoplasm of the cells of one 

 filament flows into the cells of the 

 other and there forms a spherical 

 mass, enclosed within a rather thick 

 wall, called the zygospore. The origi- 

 nal cell-walls then decay, the spores 

 ripen and from each of them a new 

 plant like the parent form will spring. 

 Watching them under the micro- 

 scope from day to day, spiral bands 

 will be seen forming in the interior 

 green mass. In due time the outer 

 covering breaks, and the young plants 

 appear as oval cells with bright-green 

 spiral bands of chlorophyll, which 

 slowly elongate, divide and produce 

 new filaments. 



We have no time this evening to 

 indicate the important role which 

 the most minute living creatures play 

 in the world ; but, owing to the inter- 

 est now taken in sanitary questions, 

 it seems advisable to briefly refer to 

 those minute plants known as bac- 

 teria, which are supposed to be the 

 active agents in the production and 

 spread of contagious diseases. 



It has been found that in the blood 

 of men and animals suffering from 

 certain disorders, of a more or less 

 fatal and contagious character, there 

 are numerous rod-like or spherical 

 particles, which are the bacteria. By 

 cultivating these organisms in suita- 

 ble fluids, such as milk, extract of 

 meat, chicken broth, etc., they can 

 be made to propagate rapidly, and 

 the progeny, after many generations, 

 will still possess the power of 

 producing the disease when 

 introduced into the blood of a 

 healthy animal. But it has also 

 been discovered that by cultivating 

 the same virulent bacteria in a certain 

 way, they can be so changed in their 

 action upon the system that they will 

 no longer produce the disease in its 

 original severity, but give rise, by in- 

 oculation, to a modified form of the 

 same disease which is mild, not in the 



