80 



BOTANY 



PART I 



chlor-ziuc-iodide. Rigidity of the thallus, especially in forms that grow exposed 

 to the surf, is provided for by thickening of the walls of the outer layers of cells 

 and sometimes by incrustation with calcium carbonate. In the Bladder Wrack 

 (Fucus vesiculosus) special mechanical cells, characterised by their thickened walls 

 and their great extensibility and elasticity, are present. The Laminarieae, which 

 are also Brown Algae, attain the highest grade of internal differentiation. In the 

 thick stem-like axis of these plants a cortex, a central body, and a loose medulla 

 can be distinguished. The cortex frequently contains mucilage canals, and the 

 medulla has rows of cells resembling sieve-tubes and serving for the transport of 



materials ; such cells also occur 

 in some Rhodophyceae. The axis 

 grows in thickness by the con- 

 tinued division of a cortical layer, 

 which forms concentric zones of 

 ^^ secondary tissue, recalling the 



" annual rings of the higher plants. 



The thalli of LICHENS arise 

 by the interweaving of fungal 

 hyphae and can assume a paren- 

 chymatous structure. The peri- 

 pheral layers in many species 

 form a protective rind owing to 

 the close association of the hyphae 

 and the thickening of their walls. 



(b) Bryophyta( 53 ). The 

 fact that the Mosses and 

 Liverworts (Bryophyta) as- 

 similate carbon dioxide finds 

 expression in their external 

 form and internal structure. 

 There are Liverworts such as 

 Eiccia fluitans (Fig. 9 1 ) in 



FIG. 90. The growing point of Dictyota dichotoma, show- i r ,-t j-V i 



ing the dichotomous branching. A, Initial cell. whlch the dichotomously- 



(x circa 500. After E. DE WILDEMAN.) branched ribbon-shaped body 



resembles the thallus of 



Dictyota (Fig. 83). In Blasia pusilla, another Liverwort (Fig. 92), the 

 ribbon-shaped thallus has a midrib and bears lateral lobes as if the 

 separation of leafy structures was commencing. The most completely 

 segmented Liverworts, such as Plagiochila asplenioides (Fig. 93), and 

 all the Mosses have cylindrical branched stems bearing such leaves as 

 organs of assimilation. The lateral branches stand beneath the leaves 

 on the main axis. These dorsiventral, bilateral, or radially-symmetrical 

 bodies, which are often in the Mosses associated in tussocks, are only 

 analogous to the shoots of the higher plants and are best regarded 

 as highly-differentiated thalli. Though these plants, in contrast to 

 the Algae, are mostly sub-aerial organisms they do not possess true 

 roots, but are attached to the soil by RHIZOIDS. These are unicellular 

 hairs, separated from the basal cell bearing them by a cross wall, or 



