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A TEXTBOOK OF THEORETICAL BOTANY 



Structure of the Thallus 



The tissues of the thallus are very simple and are parenchymatous 

 throughout, the upper and lower surfaces being formed of smaller cells, 

 but not cuticularized or otherwise distinguished beyond the fact that the 

 plastids in the upper surface layer occupy nearly the whole cell. Every cell 

 of the thallus contains one large irregularly shaped chloroplast containing 

 one pyrenoid (Fig. 393), with numerous small starch grains. The pyrenoid 

 is peculiar to this genus, and is the only case of its occurrence outside the 



Fig. 393. — Anthoceros laevis. Cell of thallus showing 

 the single large chloroplast containing stained 

 starch grains. 



Algae. The division of the chloroplast precedes cell division. The two 

 plastids move to opposite sides of the nucleus, where they behave as centro- 

 somes, the spindle fibres running between them across the nucleus. The 

 thallus is six to eight cells thick, the central layers having only small plastids 

 and no intercellular spaces. Smooth-walled unicellular rhizoids are formed 

 below the thallus. 



In its early stages the thallus grows by a single apical cell, but in mature 

 specimens there may be many, scattered along the margin and separated 

 from each other by small projections, the middle lobes, which later develop 

 into the characteristic crenulations of the margin. 



At certain places the superficial cells of the lower surface separate from 

 each other, leaving intercellular slits filled with mucilage, which oozes out 

 and holds water between thallus and soil. These slime pits deepen and 

 invade the interior tissues, forming rounded cavities filled wdth mucilage, 

 which are often inhabited by colonies of the Cyanophycean Alga, Nostoc. 



