132 PLANT LIFE. 



come in contact with the carpogonia, which become fertilized 

 by this very indirect method. A single fertilizing-tube fre- 

 quently fertilizes several carpogonia, by increasing in length 

 and passing from one to another. Several other genera 

 exhibit a similar arrangement to the above, and in many 

 instances the process is yet more complex. 



In some genera, as Polysiphonia, the carpogonia project 

 from the thallus, and are sometimes furnished with a short 

 stalk ; but in the majority of cases they are immersed in the 

 substance of the thallus, the trichogyne alone appearing 

 above the surface. 



In some species antheridia and carpogonia are produced 

 on the same plant, but by far the greater number are 

 dioecious, consequently in such cases a species includes 

 three distinct individuals, the asexual form bearing tetra- 

 spores, as a rule the most abundant ; and the two sexual 

 plants bearing antheridia and carpogonia respectively. In 

 many cases the three forms of an individual are sufficiently 

 distinct in size and form as to be recognized at once, the 

 asexual condition being very often the largest and brightest 

 coloured of the three. 



In the simple types of structure, as Batrachospermum, the 

 antheridia consist of the terminal cells of slender septate 

 branches, each antheridium containing a single poUinoid ; 

 in the higher forms the antheridia are usually produced in 

 dense clusters on or near the tips of the ultimate branchlets. 

 In Dasya coccinea they originate in the following manner. 

 The main stem is polysiphonous and corticated, but the 

 ultimate branchlets consist of a single row of large cells 

 which corresponds to the axial row in the stem. On these 

 ultimate branchlets the antheridia are formed. As in Poly- 

 siphonia, increase in length depends on the segmentation of 



