SOME TYPES OP PLOWERLESS pLANTS. 



225 



latter, are hot certain hereafter to be recognized in any strictly 

 scientific classification of cryptogamous plants. 



AlgsB vary in size from spheres xtrrVTyxr ^^^^ i° diameter 

 to great cable-like masses many hundreds of feet in length. 

 Some species are found in salt, some in brackish, some in 

 fresh water. There are species which occur growing on snow 

 and melting ice, while others form the characteristic vegeta- 

 tion of hot springs, in which they sometimes endure a tem- 

 perature nearly equal to that of boiling water. 



275. Reproduction in Algce. 

 — The reproductive processes 

 in algse are of several types, 

 which are described in special 

 treatises but cannot be ex- 

 plained in detail in a botany 

 for beginners. Besides the 

 mode by formation of zoospores, 

 as in the Protoooccus, and that 

 by the formation of zygospores, 

 as in the desmids and in Spi- 

 rogyra, there is a very interest- 

 ing method which maybe briefly 

 outlined here, because it repre- 

 sents an important principle 

 in many kinds of reproduction, 

 the union of fertilizing cells 

 with much larger egg-cells. This 

 kind of union is well illus- 

 trated by one of the very com- 

 monest of seaweeds, the common bladder-wrack or rockweed, 

 Fig. 196, which grows on rocks between high and low water 

 mark. It has many flat, leathery branches, which are buoyed 

 up in the water by the air-bladders, b. The spores are pro- 

 duced by means of a rather complicated set of organs con- 



FiO. 196. — Common Bladder-Wrack or 

 Eookweed, Fucus vesicyulosus. (Re- 

 duced to about % the natural size.) 



b, air-Wadders ; /, organs for produc- 

 tion of spores. 



