614 PHYSIOLOGY. 



thallus, as we see it practised in propagating the Mushroom, the Vinegar- 

 plant, &c. In the Lichens there is a proper structure to which the vege- 

 tative mutiplication is confined, viz. the yonidia, the green cells formed in 

 the medullary layer of the thallus, which frequently break out from the 

 surface and become free, especially when the plants are exposed to exces- 

 sive damp. These gonidia, however, are now considered to be Algae, 

 entangled in the meshes of a parasitic fungus. 



In the Algae the vegetative multiplication exhibits very varied charac- 

 ters. In the Confervoids (p. 451) we have the zoospores and swarm-spores 

 (fig. 512, C, d, and fig. 591), as also in the Phaeosporeae : and the tetraspores 

 of the Rhodospermeae and the Dictyotaceae probably have the same im- 

 port ; but, in addition to this, the thallus is commonly multiplied, especially 

 in the larger forms, by the growth of a number of new thalli from the 

 sides or the base of an old plant, and their subsequent separation by the 

 decay of the parent thallus. 



In the above cases we see the double representation of the vegetative 

 process which occurs in so marked a manner in the higher plants. We 

 have increase by simple and pure subdivision of ordinary vegetative 

 structures, and, besides this, we see varied modifications of the vegetative 

 cells specially organized to fit them for being thrown off spontaneously 

 (i)(>mdia. &c.). 



In the Hepaticae and Mosses the propagative structures do not arrive 

 at the condition of bttds, although the parent plants have leafy stems. 

 The gemmce of these Classes are merely cellular nodules, more or less de- 

 veloped in different cases, and only acquire leaves after they have become 

 independent. In the Jungermanniaceae they are developed on the leaves 

 or in place of fruits. In Marchantiacese they are fouitd in cup-like re- 

 ceptacles, being especially frequent when the plants grow in damp, shaded 

 localities, a number of them (springing originally from a single cell) lying 

 in the cup like eggs in a nest. 



The Mosses produce gemmce from all parts of their structures from 

 their leaves, stems, metamorphosed fruit-organs, and, above all, from 

 thread-like runners (protonema) which shoot out from the base of their 

 stems. When their spores germinate, they also form first a mesh of 

 confervoid filaments, each joint of which often gives birth to a leaf-bud 

 (fig. 499, p. 430). 



Buds of Vascular Cryptogams. The Ferns and allied Classes agree 

 more closely with the Flowering plants in their vegetative propagation, 

 forming leaf -buds in cases where they increase in this way ; but there is 

 a connexion with the Mosses c. in the circumstance that their gemma 

 appear more frequently on the leaves than is the case normally in the 

 Phanerogamia as, for example, in Asplenium rhizophyllum, where the 

 leaves root and form buds at their tips, Cystopteris bidbifera, in which 

 bulbils appear on the petiole, &c. 



Buds of Phanerogams. In the Phanerogamia the rule is, that every 

 leaf -bud may be separated from the parent stock, and, if properly treated, 

 reared into a new plant ; moreover, in a vast number of cases, the leaf- 

 buds are naturally modified in certain details of their structure, so as to 

 protect them from external injury, and then thrown off spontaneously by 

 the parents to multiply the kind/ Many of the cases of this phenomenon 

 have been described 'in the first part of this work under the head of 



