OOPHTTA. 147 



(h) Make very thin cross and longitudinal sections of different por- 

 tions of the plant-body, and study the tissues. Note particularly the 

 boundary tissue (epidermis), and the cells constituting the midribs 

 and harder portions of the stems and leaves. 



(e) The following key to the genera of American Fucacese will be 

 helpful in their study. 



I. Plant branched: 



1. Leafy; air-bladders stalked, separate Sargassum. 



In addition to half a dozen species of both coasts, the 

 Gulfweed (Sargassum bacclferum) may be mentioned, 

 which floats in great quantity in raid-Atlantic, consti- 

 tuting the so-called Sargasso Sea. Its proper home is in 

 the West Indian region, where it grows attached to rocks. 



2. Leaves spiriilly inserted, bearing air-bladders on their 



blades (Soulhern) .Turbinaria. 



3. Leaves 3-ranked, bearing air-bladders on their petioles 



(Western) Phyllospora. 



4. Plant piimatifid; air-bladders several celled, terminal on 



the brauchlets (Western) '.Halidrys. 



5. Plant dichotomous, the parts flat and provided with a mid- 



rib (both coasts) Fucus. 



This contains the proper Rockweeds of the seaside. 

 Eight species occur in the United States. 



6. Plant irregularly dichotomous, the linear parts destitute 



of a midrib (Eastern) Ascophyllum. 



7. Plant much branched, bushy, the branches filiform (West- 



ern) Cystoseira. 



11. Plant reduced to a top-shaped or cup-shaped vesicle (doubt- 

 fully American) Himanthalia. 



(d) The filamentous oSphytes may be preserved for herbarium 

 specimens by floating out as described on page 129 (J). The Mildews 

 and White Rusts may be preserved by simply drying the affected 

 leaves and stems (of the hosts) under pressure. 



(e) Preserve specimens in weak alcohol for future study. 



