HOW TO KNOW THE FRESH-WATER ALGAE 



Figure 286 



399a Sheath containing 2 or more trichomes. Fig. 286. . .SACCONEMA 



Fig. 286. Sacconema rupes- 

 tre Borzi. a, habit of col- 

 ony; b, filaments from col- 

 ony. 



Trichomes in this genus 

 are tapering from a basal 

 heterocyst as in Gloeotrichia 

 (Fig. 287) but there is more 

 than 1 trichome within a 

 sheath and the gelatinous 

 colony is very irregular in 

 shape as it occurs on stones 

 (sometimes in very deep water). The sheaths are wide, lamellate and 

 are flaring at the outer end. 



399b Sheath containing 1 trichome 400 



400a With cylindrical spores adjoining a basal heterocyst; colonial 

 mucilage soft in floating species, firm when attached to substrates 

 and then forming hemispherical growth 1-3 mm. in diameter. 

 Fig. 287 GLOEOTRICHIA 



Fig. 287. a, Gloeothrichia Pisum (Ag.) Thur., 

 habit on Ceratophyllum; b, diagram of fila- 

 ment arrangement; c, G. echinulata (J. E. 

 Smith) P. Richter, diagram of filaments in 

 colony; d, diagram of base of single fila- 

 ment showing heterocyst and spore. 

 In this genus the tapering trichomes are 

 encased in mucilage which is usually soft (al- 

 though firm in 1 species) and they are not so 

 closely compacted as in Rivularia (Fig. 288), 

 a genus which is very similar. Gloeotrichia 

 produces filaments with large akinetes 

 (spores) adjoining the basal heterocyst when 

 mature. One of the most common species 

 is the free-floating G. echinulata which occurs 

 in abundance in some lakes and gives the 

 appearance of the water being filled with 

 tiny tapioca-like grains, buff-colored. G. na- 

 tans (Hedw.) Rab. is also fairly common. It begins as an attached 

 plant but late in the growing season it appears at the surface in brown 

 gelatinous masses, either expanded and flat or irregularly globular. 

 G. Pisum forms hard, green or black balls, 1 or 2 mm. in diameter on 

 submerged vegetation, sometimes completely covering the host plant. 

 When plants in this genus are young the spores will not be present, in 

 which case they may be confused with Rivularia (Fig. 288) which never 

 produces spores, has a much harder colonial mucilage and has tri- 

 chomes more densely packed, often parallel in arrangement. 



Figure 287 



166 



