PLATE XXVL— SEXUAL PROCESS TRACED FROM MOULD TO FLOWERING PLANT. 



In dealing with the life histories of organisms, it has already been shown that the simplest form of the sexual process obtains in 

 Mucor. A bud from one hypha grows out to meet a bud from another hypha, and some mysterious attraction brings them together. 

 The ends of the two blend and become parted off to form a single body, which is thus the result of a process of Conjugation. In 

 Ulothrix, likewise, the sexual process is, if possible, simpler. The contents of a cell, instead of growing out, breaks up into small 

 particles, which round themselves off and acquire two cilia, by means of which they move about. When free in the water two moving 

 particles from different cells meet and blend to form one body, as in Mucor. 



These two forms may be taken as starting-points for tracing the sexual process in plants ; the one representing the condition where 

 the conjugating elements are passive and the plant is without chlorophyll, the other where the conjugating elements are active and the 

 plant possesses chlorophyll. 



It will not be necessary to go over each life history in detail, for that has been done to a certain extent already, but simply to point 

 out the changes taking place, both in the sexual elements themselves and in their mode of blending, in passing from the lowest to the 

 highest term of the series. It will be convenient to distinguish two principal stages in the life history of each plant — a stage with 

 sexual organs called the Sexual Generation, and a stage with no sexual organs called the Non-sexual Generation. The Sexual Generation 

 opens the chapter, and a return to that ends it On glancing over the Diagrams it will be seen that the Sexual Generation is gradually 

 suppressed, until in the Flowering Plant it is microscopic in its dimensions, and reduced to a few cells, while the Non-sexual Generation 

 grows in importance, becoming the stately tree or the conspicuous flowering plant. 



Note. — In this comparative view the term Spore is used in a different sense from that in the body of the work. In the lower 

 forms it is a cell resulting from a sexual process, viz. Zygospore or Oospore, and it is restricted to that throughout the series. Other 

 cells which multiply the plant are Gonidia, although from Liverwort onwards they are usually called Spores. 



Mucor. — The Sexual Generation is the conspicuous mould, producing the similar and stationary male and female elements, which 

 blend to form a Zygospore. 



The Non-sexual Generation is the inconspicuous Pro-mycelium, consisting of a single hypha, which produces Endo-gonidia, from the 

 germination of which Mucor is reproduced. 



Ulothrix. — The Sexual Generation is the filamentous Alga, producing similar and motile male and female elements, which blend 

 to form a Zygospore. 



The Non-sexual Generation is the Zoogonidia, derived directly from the internal division of the Zygospore, and reproducing the 

 filament 



Oedogonium. — The Sexual Generation is the filamentous Alga, producing no longer sexual elements which are alike or nearly so, 

 but now clearly distinguishable — Antherozoids and Oospheres — the Oosphere becoming converted into an Oospore by the impregnation 

 of the Antherozoids. 



The Non-sexual Generation is similar to that of Ulothrix. 



Chara. — The Sexual Generation is the plant with whorled appendages, producing Antherozoids and Oospheres, from which result 

 Oospores. 



The Non-sexual Generation is the Pro-embryo, which gives rise to a bud producing the plant 



Marchantia. — The Sexual Generation is the conspicuous green expansion, producing Antherozoids on the male plant, and Oospheres 

 on the female plant The Oosphere, or central cell of the Archegonium, is impregnated by Antherozoids, and converted into an 

 Oospore. 



The Non-sexual Generation is the Spore-fruit, producing Endo-gonidia, from the germination of which Marchantia is produced. 



Moss. — Similar to Marchantia, only in the Non-sexual Generation the germinating Endogonidium does not directly produce the 

 Moss, but a Protonema is formed, from which a lateral bud arises and grows into the plant. 



Fern. — The Sexual Generation is the Prothallus, a minute, green, heart-shaped expansion, corresponding to the leafy Moss. This 

 produces Antherozoids and Oospheres, which latterly become Oospores. 



The Non-sexual Generation is the Fern, corresponding to the Spore-fruit of the Moss. This developes Endo-gonidia, each of which 

 produces on germination a Protonema. The Protonema is a row of green cells, often branched like that of the Moss, and afterwards 

 developing into the Prothallus. 



Eqtjisetum. — The Sexual Generation is the Prothallus, male and female distinct 



The Non-sexual Generation is the Equisetum, the fertile shoots of which repeat the history of the Fern. 



Selaginella. — The Sexual Generation is the Prothallus, male and female distinct and internal. 



The Non-sexual Generation is the Selaginella, which has in its embryonic condition a special structure called the Suspensor. 



Pi NE , — The Sexual Generation is represented by the Male Prothallus, or cells forming the full-grown Pollen-grain, and the Female 

 Prothallus, or Endosperm. 



The Male Prothallus is exceedingly simplified. There is only one or a few cells to represent the vegetative part, and a single 

 large cell to represent the antheridial part, or the part which formerly produced Antherozoids. The production of Antherozoids was 

 suitable for plants living in moist situations; but as Conifers live in dry situations, Antherozoids would fail of their purpose, and the 

 nuclei do not develope cilia for locomotion. So the representative of the antheridial cell puts forth a pollen-tube, along which the 

 nuclei are conveyed to their destination, viz. the germ-cell. 



The Female Prothallus is represented by the Endosperm, in which the germ-cells are developed. 



The Non-sexual Generation is the tree quite comparable with Selaginella, the Embryo-sac corresponding to the Macrogonidium, and 

 the young Pollen-grain to the Microgonidium. 



Flowering Plant. — The Sexual Generation is the Male Prothallus, or cells forming the mature Pollen-grain, and the Female 

 Prothallus, or contents of Embryo-sac. 



The Non-sexual Generation is the conspicuous Flowering Plant, producing Pollen-grains and Embryo-sac. The modified leaves of 

 the Flower — Stamens and Carpels — which produce Pollen-grains and Embryo-sacs are usually called Sexual Organs; but they are really 

 equivalent to the fertile leaves of Selaginella, the Pollen-grains being Micro-gonidia in Pollen-sacs, and the Embryo-sacs being Macro- 

 gonidia in Ovules. Cells are afterwards developed in the interior of Pollen-grain and Embryo-sac, which represent the Sexual 

 Generation. 



Multiplication takes place in each case by a smaller or larger portion of the plant detaching itself and growing to the size and 

 form of the parent The directness and simplicity of this process are evident in the highest as well as in the lowest forms. 



Fig. 1. Pollen-grain of Larch consisting of several cells. 



Fig. 2. Pollen-grain of Monotropa (Dicotyledon). 



(a.) Young Pollen-grain consisting of two nucleated cells. 

 {b.) Pollen-tube formed containing the two nuclei. 



