124 Bvolutwn of Verjetal Life. 



is retained, as we have seen in the propagation by cuttings. 

 A more marked instance is to be found in the case of the 

 begonia, which the florists propagate from fragments of 

 the leaves. In some of the garden lilies, perfect bulbs are 

 formed in the axils of the leaves ; in other plants, bulbs 

 are formed under ground; in the potato, large stems or 

 tubers are formed upon the roots, and from these new plants 

 are grown. 



In the simplest plants formed of aggregations of cells, 

 the independence of the cells seems to be simply limited by 

 their physical attachment to each other ; subsequently the 

 functions of the parts become diversified, and a division of 

 labor begins. 



The next form of the reproductive process is found in the 

 desmids and diatoms, which, beside multiplying by division, 

 also multiply by conjugation ; that is, two cells or plants 

 unite to form a compound plant, the contents of which, tak- 

 ing upon itself a coat of cellulose, begins a new series of 

 individuals. The next is a differentiation in the structure 

 of the plant by means of which certain cells called anther- 

 idia, produce antherozoids, which correspond with the grains 

 of pollen in the higher plants, and other cells produce 

 ob'spheres which correspond with the protoplasmic contents 

 of the ovules. Then we come to the mosses, which have pro- 

 cesses resembling pistils and antheridia, either on the same, 

 or on separate plants. Then ferns, in which the spores, 

 found upon the underside of the fronds or leaves, drop off 

 when ripe, and produce minute plantlets, which contain sep- 

 arate elements which must unite before fertilization is ef- 

 fected. Above this grade are the flowering plants, in which 

 is more or less developed the complete system which I have 

 described in the case of the rose. 



In much the greater number of the plants with which we 

 are familiar, the stamens and pistils are found in the same 

 flower ; sometimes they are found in different flowers upon 

 the same plant ; sometimes some of the flowers upon a.plant 

 are perfect, and some are staminate or pistillate only ; and 

 sometimes the staminate or pistillate flowers are found upon 

 separate plants. The last form is esteemed the most high- 

 ly developed. I should note in passing that while in all 

 the higher orders of plants having flowers with stamens and 

 pistils, fertilization seems to be necessary for the main- 

 tenance of the race, nevertheless instances have been occa- 



