LECTURE V. 



METAMORPHOSED AND REDUCED FORMS OF SHOOT OF THE 

 VASCULAR PLANTS. THE SHOOTS OF MOSSES, ALG.^, AND FUNGI 



The further, with progressive division of labour in the body of a plant, the 

 particular biological r6le assigned to a shoot becomes removed from that of the 

 typical leaf-shoot, the more its external form, internal structure, and irritability also 

 are modified. A deeper study of these metamorphoses and degenerations of the 

 shoot might easily lead to a very long series of monographs of the most various 

 plants; we must content ourselves, however, with a condensed survey of the more 

 common cases. 



Let us first consider some modified forms of sub-aerial shoots, among which 

 those of the so-called Succulents are distinguished in that, while they possess 

 massive shoot-axes or leaves, a surprisingly small extension of the surface is 

 produced : in contrast with the slender, graceful structure of the typical leaf- 

 shoot, we have thus to do here with stout, massive forms. By means of this 

 constitution, the succulent plants are enabled to live, even in hot dry air, with a 

 small supply of water from the earth; of course at the expense of their increase 

 in body-weight, since the small development of surface allows such plants only a 

 relatively inconsiderable superficial extension of the chlorophyll, and accordingly 

 a less plentiful production of organic substance. 



Confining ourselves first to the very decided cases of succulence, this may 

 appear either in the shoot-axis or in the leaves. 



In the succulent shoot-axes, the leaves, although formed at the growing point, 

 usually become partly or wholly arrested later; the developed shoot then consists 

 only of the leafless axis. This is the case in the true Cactus forms, Echinocactusfi, 

 Mamillaria, Cereus, Opuntia, &c., the shoot-axes of which resemble either roundish 

 tubers or thick prismatic bodies, on the surface of which the places whence the 

 leaves have disappeared are marked by tufts of hairs and prickles, situated on pro- 

 jecting cushions or ridges. Occasionally, the shoot-axes of Cac/i also assume flat, 

 leaf-like forms (Phylloiadus, &c.). In other families, succulent leafless shoot-axes are 

 found in individual species, the habitats of which are favourable to such an arrange- 

 ment. Thus we have Cactus-like forms in the family of the Asclepiadaceae in the 

 African genus Siapelia ; and in the genus Euphorbia in a series of species likewise 



