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! 
. 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 
openand 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 
v:¢) note their peculiarities. Sketch a portion 
FiG. 183. — Part of Thallus of 
a Rockweed (Fucus platy- 
carpus), naturalsize. The 
two uppermost branchlets 
are fertile. 
4) 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 papille (concep- 
tacles) as they appear in this section, and 
make a simple sketch to show their position. 
Select some plants with brighter colored 
tips and some less bright, if any difference 
1 Fucus vesiculosus is the most available species. 
Others may be substituted. 
