THE SEA-LETTUCE 39 



Thus far in our study of the phenomena of sex we 

 have considered the auxospore of the Diatoms and the 

 zygote of the Desmids, both of which result from the 

 fusion of sexually undifferentiated cells; but mSpirogyra 

 we detect some signs of sexual difference in the conju- 

 gating cells. The Conjugatse, however, are regarded 

 as a side-line in evolution, and if we wish to study sex 

 in its further advances, we must turn to plants of another 

 class. We need not, however, go beyond the limits of 

 the Algse to discover new manifestations. Still con- 

 fining ourselves to the Green Algse (Chlorophycese), we 

 shall find in the class Confervoide^ some plants which 

 will yield us some interesting evidence. 



Before we proceed, we must familiarize ourselves with 

 the term " thallus." This word is from the Greek 

 thallos, a young shoot, and botanists apply it to the 

 vegetative body of a plant which shows little or no 

 differentiation into root, stem, or leaf. A Spirogyra 

 filament is a thallus. All the plants we have hitherto 

 considered are thallophytes, and we shall continue to 

 treat of them until we have disposed of the Fungi, which 

 are included among them. 



The thallus of the Confervoidese is always multi- 

 cellular. The cells are usually arranged in a linear way, 

 to form either branched or unbranched filaments. Yet 

 the thallus does not always consist of a filament formed 

 of a single row of cells, for it may be composed of cell- 

 masses, or cells united into a leaf-like form. The green 

 seaweed known as the Sea-Lettuce, or Green Laver 

 {Ulva latissima), belongs to this class. Its thallus is 

 composed of two layers of cells, and occurs in the form 

 of good-sized wavy fronds. Enteromorpha is another 



