486 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



the circumstances influencing the persistence of 

 their vitality and the vigor of their growth. 



The theory of biogenesis has also lately been 

 the guide in the investigation of the causes of 

 various forms of disease, both in the lower ani- 

 mals and in man, with the result of showing that 

 in many of them the infective substance consists 

 in all probability of germs of minute animal or 

 vegetable organisms. 



There i3 very great probability, indeed, that 

 all the zymotic diseases, by which we understand 

 the various forms of fevers, have a similar origin. 

 As has been well remarked by Baxter in an able 

 paper on " The Action of Disinfectants," the 

 analogies of action of contagia are similar to 

 those of septic organisms, not to processes simply 

 of oxidation or deoxidation. These organisms, 

 studied in suitable fluids, multiply indefinitely 

 when introduced in all but infinitesimal propor- 

 tions. Thus they are, as near as we can per- 

 ceive, the very essence of contagia. 1 



Leaving, however, these and many other gen- 

 eral questions regarding the origin of the lowest 

 forms of animal and vegetable life, let us now 

 turn our attention to the mode of development 

 of a new being in those belonging to the higher 

 groups. The general nature of the formative 

 process, in all instances where fertilized germs 

 are produced, will be best understood by a short 

 sketch of the phenomena ascertained to occur in 

 different kinds of plants. 



In the higher or phanerogamic plants it is 

 generally well known that the combination of 

 two parts of the flower is necessary to the pro- 

 duction of a seed containing the embryo or young 

 plant. Beginning with the discovery of the pol- 

 len-tubes by Amici in 1823, the careful and 

 minute investigations of a long line of illustrious 

 vegetable physiologists have brought to light the 

 details of the process by which fertilization is 

 effected, and have shown in fact how the minute 

 tube developed from the inner membrane of the 

 pollen-granule, as soon as it falls upon the stig- 

 matic tissue of the seed-bearing plant, insinuates 

 itself by a rapid process of development between 

 the cells of the style, and reaches at last the 

 ovule, in the interior of which is the embryo-sac ; 

 how, having passed into the micropyle, or orifice 

 of the ovule, it makes its way to the embryo-sac ; 

 how a minute portion of the fertilizing substance 

 of the fovilla transudes from the pollen-tube into 



1 For the most interesting information on this subject, 

 I cannot do better than refer to the very able reports by 

 Dr. Burdon-Sanderson in the "Reports of the Medical 

 Officer of the Privy Council," 1873, 18T4, and 18T5. 



the cavity of the embryo-sac, in which by this 

 time a certain portion of the protoplasm has be- 

 come differentiated into the germinal vesicle, 

 thereby stimulating it to further growth and de- 

 velopment, the earliest phenomena of which mani- 

 fest themselves by the formation of an investing 

 cell-wall, and by the occurrence of cell-division, 

 which results in the formation of the embryo or 

 plantule of the seed. 



Thus it appears that the essential part of the 

 process of production in phanerogamic plants is 

 the formation in the parent plant of cells of tw# 

 different kinds, which by themselves have little 

 or no independent power of further growth, but 

 which, by their union, give rise to a product in 

 which the power of development is raised to the 

 highest degree. 



By further researches it is now known that 

 the same law prevails in all the remaining mem- 

 bers of the vegetable kingdom, with the excep- 

 tion only of the very simplest forms. 1 



In viewing the reproductive process in the 

 series of eryptogamic plants, two facts at once 

 strike us as remarkable in the modifications which 

 are observed to accompany the formation of a 

 productive germ, namely, first, that the differ- 

 ence between the two productive elements be- 

 comes, as it were, more prominent, or more high- 

 ly specialized, in the eryptogamic than in the 

 phanerogamic plants; and, second, that in the 

 simpler and lower forms this difference gradually 

 disappears till it is lost in complete uniformity of 

 the productive elements. 



Thus, in the whole tribe of the ferns and vas- 

 cular cryptogams, in the higher algae and fungi, 

 in the characese and in the mosses, the differen- 

 tiation of the productive elements is carried to a 

 very high degree ; for, while that belonging to 

 the embryo or germ presents the structure of a 

 simple cell which remains at rest, or in a com- 

 paratively passive state, and, absorbing into it- 

 self the substance of the other, becomes the seat 

 of^subsequent development the, other, corre- 

 sponding to the pollen of the staminiferous pha- 

 nerogam, is usually separated from the place of 

 its formation, and, having undergone a peculiar 

 modification of structure by which it acquires 

 active moving cilia, it changes place, and is di- 

 rected toward the germinal structure, and, com- 

 ing in contact with its elementary cell, is more 

 or less absorbed, or lost in the fertilizing process. 

 The protoplasm of the germinal cell, thus acted 



1 It will be observed that I leave entirely out of view 

 the whole subject of the multiplication of plants by bud- 

 ding or simple division. 



