250 



FOUNDATIONS OF BOTANY 



mode of growth. The stem has nodes and internodes. 

 The stem increases by the growth of an apical cell, but 

 growth in length depends chiefly on the elongation of each 

 internodal cell instead of the multiplication of numerous 

 internodal cells. 



THE STUDY OF ROCKWEED 1 



291. Occurrence. The common rockweed is abundant every- 

 where on rocks, between high and low tide, on the New England 

 coast and southward. 



292. The Frond. A plant of rockweed 

 consists mostly of a growth which is some- 

 what leaf-like, but, in fact, stem and leaf 

 are not separately developed, and the growth 

 is therefore called a thallus. This combined 

 stem and leaf has many flat leathery 

 branches which are buoyed up in the water 

 by air-bladders. Cut one of the bladders 

 open and note its form and appearance. Note 

 whether they occur singly or how grouped. 

 Note the prominent midrib running through- 

 out the middle of each branch. Examine 

 the swollen tips of some of the branches and 

 note their peculiarities. Sketch a portion 

 of a frond to show the characteristics so far 

 noted. 



293. Reproduction. Cut across through 

 the middle of one of the swollen fruiting 

 tips. Note the fruiting papillae (concep- 

 tacles) as they appear in this section, and 

 make a simple sketch to show their position. 



Select some plants with brighter colored 



FIG. 183. Part of Thallus of tips and some less bright, if any difference 

 a Kockweed (Fucus platy- 



carpus) , natural size. The 

 two uppermost branchlets 

 are fertile. 



1 Fucus vesiculosus is the most available species. 

 Others may be substituted. 



