SEXUAL EEPEODUCTION. 621 



Further, reproduction is an exhausting process ; it kills some animals : 

 and excessive fruiting exhausts perennial plants. 



At the same time, the reproductive tendency and the vegetative ten- 

 dency appear contrasted and opposed to each other ; for reproduction is 

 often retarded and replaced by rapid development of vegetative structures 

 when plants are placed in too favourable a soil, especially when too freely 

 supplied with water ; and rankly growing plants are frequently made to 

 flower by gardeners cutting the roots, confining them to small pots, or 

 limiting the supply of water. 



The reproductive bodies produced by plants are either developed at 

 certain epochs from structures originally belonging to the vegetative 

 system, or they are formed in special organs. In the lower Algae we find 

 the cells, as those of the filaments of (Edogonium (fig. 505) or Spirogyra 

 (fig. 5 12), originally true vegetative cells, and at a certain stage of grow y th 

 resolved into reproductive cells and producing spores from their green 

 contents. As we rise in the scale, among the Thallophytes, we soon find 

 special cells (Penicittium, &c., fig. 1, C) or groups of cells, exclusively 

 vegetative or exclusively reproductive. In the higher Cryptogams, 

 assemblages of organs of various kinds are formed upon the stems, in 

 which are ultimately ripened the spores of this group ; while in the highest 

 class, the Phanerogamia, we meet with fiowers containing stamens and 

 pistils, ultimately producing true seeds in fruits which are totally separated 

 m almost every case from the vegetative structures. 



The spores of the higher Cryptogams (Ferns, Mosses, &c.) cannot be 

 properly compared to the seeds of the Flowering plants (that is, morpho- 

 logically), since they result from a series of physiological processes different 

 in many respects and not directly dependent on sexual agency. With 

 regard to the spores of the Thallophytes, our knowledge is too imperfect 

 at present to enable us to decide in all cases upon all the homologies ; the 

 probable relations of the different kinds of structure are incidentally 

 spoken of in the Sections devoted to the description of these plants. 



It is probable that representatives of two sexes, male and female, exist 

 in all plants, and that these conjoin to form the rudiments of the new in- 

 dividuals of all Cryptogams, as they do in the formation of the embryo 

 in the seed of Phanerogams. But in the Thallophytes the male and 

 female organs are often reduced to simple masses of protoplasm, " sperm- 

 cell" and "germ-cell " (these being associated often in the same plant), 

 bud-cells, conidia, &c. serving the purposes of vegetative propagation : the 

 exact particulars and homologies are still obscure in many families. 



The history of reproduction of plants has been greatly studied and much 

 enlarged of late years ; many important discoveries have been made in all 

 classes ; and the course of the processes in Phanerogams and the leafy 

 Cryptogams is now pretty well known. Much still remains to be dis- 

 covered in reference to the Thallophytes, especially the Fungi ; but in 

 the Algae the processes of fertilization of germ-cells by spermatic cor- 

 puscles have been observed more clearly and definitely than in any 

 other plants. 



Conjugation. The simplest form of sexual reproduction is that 

 known as conjugation, or the fusion of two masses of protoplasm 

 the one into the other, as has been already mentioned under the 



