Page:Popular Science Monthly Volume 25.djvu/170

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tached from the parent-plant. The offspring may, or may not, be like the parent. Plants which result from similar cells developed in the same manner belong to the same generation. If the original reproductive cells grow into plants without any union with other cells, these plants are asexually produced. If the union of the contents of two or more cells is necessary for further development, there results a sexual product.

It is the purpose of this paper to trace the relative size of the two generations above described, in a number of the higher orders of plants. In doing this, the fact of the alternation will be developed. The series of orders will begin with the humble Hepaticæ, and end with the most highly developed of flowering plants. The Hepaticæ, or liverworts, are small, flowerless plants of very simple structure, which grow for the most part in moist places upon the bark of trees, surface of long-exposed rocks, earth, etc. One of the leading genera is Marchantia species of which abound on the earth of flower-pots in greenhouses and elsewhere. The leaf-like expansion or thallus is the sexual generation, and bears the male and female organs in depressions of the surface. The male parts, called antheridia, produce spermatozoids, which are spiral, slender bodies, provided with two motile hairs or cilia, as locomotive appendages. The female organs (archegonia) are at first single cells, which by division form flask-like structures, the lower cell of which is the female germ-cell. When this germ is fertilized by the antherozoids, which enter at the neck of the "flask," it undergoes a development, varying somewhat in the different orders, but essentially a sporangium or spore-case is produced, in which are very many spores and slender spiral threads arranged in rows. This sporangium is the second and asexual generation of the liver-wort. The complexity of the structure of

PSM V25 D170 Marchantia polymorpha and plagiochila asplenioides.jpg
Fig. 1. Fig. 2.

the first or leaf-like generation and of the sexual organs and sporangia increases in the Hepaticæ group in passing from the lower to the higher forms. In the highest group there are stems with leaves arranged in