118 



BOTANY. 



creeks and rivers we have an example of a filamentous plant 

 of the class Zoosporese. It is a large, dark-green, much- 

 branched plant, which attaches itself to stones and timbers 

 in the water. It grows so vigorously that it soon forms long 

 matted masses, often several metres in length, which float 

 and wave back and forth in the currents of water. It pro- 

 duces myriads of zoospores. 

 246. The Sea-Lettuce (Ulva), which is common along the 

 coast and in brackish waters, grow- 

 ing upon stones, wharf -timbers, etc., 

 and resembling small lettuce-leaves, 

 reproduces by zoospores. The 

 plant is composed of a couple of 

 layers of cells, and in some of these, 

 by internal cell-formation, zoospores 

 are produced; these escape into the 

 water, where they swim about by 

 means of their two cilia. 



247. Kelp and its Allies (Phseo- 

 sporese) make up a large group of 

 zoospore-bearing plants. They are all marine, often attain 

 a great size, and are of an olive-brown color. They con- 

 stitute the Kelp which is often so abundant on the sea- 

 shore after a storm. 



248. The large, flat, leaf -like kelp (Laminaria, commonly 

 called Devil's Apron) may be taken to illustrate the larger 

 forms. The " leaf " portion is sometimes from one to six 

 metres long and nearly a metre in breadth, while its stalk 

 sometimes attains a length of two to four metres. It is 

 held to rocks and stones at or below low-water mark by 

 means of root-like processes. 



249. The zoospores, which have two cilia, are produced 



Fio. 51.— A plant o( Sea^let- 

 tuce (Ulva lactuca). Natural 



