VEGETABLE PHYSIOLOGY. 



tube as indicating a new analogy between the 

 pollen-grain and the spore of cryptogamic plants ; 

 for it is, in fact, a process of germination. The 

 length of time required for the growth of the 

 pollen-tube down to the ovule, is very various in 

 different plants, and by no means depends upon 

 the length of the style ; thus, in the night-flowering 

 cactus, which has a style from eight to nine inches 

 long, the pollen-tube reaches the ovules a few 

 hours after the pollen has been applied to the 

 stigma ; while in the pine, where there is no 

 proper style, a whole year elapses before the 

 pollen-tubes reach their destination. The lower 

 extremity of the pollen-tube ultimately comes into 

 contact with the ovule, and enters the foramen 

 or micropyle of the latter, so as to reach the 

 embryo sac pre-existing there. The result of 

 the access of the pollen-tube to the ovule, is 

 the production of an embryo in the latter, and 

 the ovule, with its contained embryo, ultimately 

 ripens into a perfect seed. Fertilisation with 

 pollen is essential for the production of fertile 

 seeds, although the ovule exists previous to the 

 act. Fruits may, in some instances, swell and 

 ripen without any process of fertilisation ; but in 

 that case, they will not contain seeds capable of 

 producing new plants. 



It is not essential for success in this process 

 that the pollen should fall immediately from the 

 anthers upon the stigma, for we have several 

 historical facts which indicate that pollen may 

 retain its vitality unimpaired, in the manner of 

 seeds. Thus, in the cultivation of the date-tree, 

 it is necessary to bring the fruitful plants under 

 the influence of the male flowers ; but on one 

 occasion, during a civil war in Persia, the male 

 date-trees of a whole province were cut down by 

 the invading troops, that the fructification of the 

 fertile trees might be prevented, and the season's 

 crop thus destroyed. But the inhabitants, appre- 

 hending such a result, had been careful previously 

 to gather the pollen, which they preserved in 

 closed vessels, and thus were enabled to impreg- 

 nate their trees when the country was freed from 

 the destroying enemy. The pollen-grains of the 

 date and of the European palm (Chamcerops 

 humilis) are said to have retained vitality after 

 the lapse of eighteen years. 



Seed-vessels are various in form as, for example, 

 in the case of the pea (a), the vessel is a legume or 



I 



;od ; in the apple (), it is a pome; and in the 

 Ibert, a nut. 



All our esculent fruits are in reality so many 

 vessels or receptacles for the seeds ; and the 

 various forms in which they appear are individ- 

 ually suitable to the purposes of their growth. As 

 we have already indicated, the seed contains the 

 embryo, or germ, of the future plant, which is 

 generally surrounded by a nutritious substance 



termed the albumen, destined for the support of 

 the young plant before its organs are sufficiently 

 matured to allow of its supporting itself. This 

 albumen varies very much in quantity, sometimes 

 being much smaller than the embryo ; while in 

 other cases, as in the coco-nut, it weighs as many 

 or more ounces than the embryo does grains. Its 

 texture is variable. It is generally fleshy, as in 

 the pea and bean ; but sometimes it is farinaceous 

 or floury, as in the wheat ; at other times it is oily, 

 as in linseed; horny, as in the coffee; or even 

 stony, as in the vegetable ivory palm. If the 

 embryo consists of one seed-lobe or cotyledon, as 

 the wheat, it is said to be monocotyledonous ; if of 

 two, as in the beech and oak, dicotyledonous and 

 these terms are generally used respectively for 

 endogenous and exogenous; while cryptogamous, 

 or flowerless plants, from being propagated by 

 spores instead of seeds, are said to be acotyle- 

 donous that is, without any cotyledon whatever. 



FRUCTIFICATION OF FLOWERLESS PLANTS. 



As already stated, the lowest forms in which 

 plants make their appearance are those of the 

 cryptogamous, or flowerless orders such as the 

 ferns, lichens, mosses, sea- weeds, and fungi. In 

 these, the mode of fructification is very remark- 

 able, and quite different from that of flowering- 

 plants. They have neither flowers nor proper 

 seeds, but are propagated by minute unicellular 

 bodies called spores. 



Ferns. In the ferns (Filices), which are the 

 largest and most highly organised of the flowerless 

 orders, little brown patches, called son', may be 

 seen on the under sides of the leaves or fronds (see 

 figs.). Each of these is composed of a number 



Ferns, shewing the Sori on the back of the Fronds. 



of minute membranous capsules (thecce), which 

 contain the reproductive spores, and which are 

 often furnished with an elastic ring, for assisting 

 in rupturing the spore-case, and thus facilitating the 

 dispersion of the spores. These spores are not 

 the result of a process of fecundation similar to 

 what has been described in the case of flowering- 

 plants. Impregnation takes place, not upon the 

 mature frond, but upon the infant fern, while as 

 yet scarcely visible to the naked eye. The spores, 

 when scattered over the soil, give rise to minute 

 cellular expansions of tissue resembling liverworts, 

 upon which male and female bodies, called respect- 

 ively antheridia and archegonia, are produced. 

 The former emit spermatozoids, which move about 

 freely, by means of attached cilia. The arche- 

 gonia have a central canal, leading down to a 

 large globular cell. A spermatozoid enters the 

 archegonial canal, reaches the globular cell, and 



