DIV. 1 



MORPHOLOGY 



75 



in intercalary growth is a zone between the base and apex, while in 

 apical growth it is situated at the summit of the plant body. A 

 young plant of the green seaweed Ulva lactuca affords an example 

 of the latter condition (Fig. 81). 



4. Flattening. Many Algae and Lichens have a disc-shaped or 

 ribbon-shaped thallus (Fig. 83) by which the free surface is further 

 increased. The assumption of this form may therefore be regarded as 

 an adaptation to the nutritive relations of the organism. The latter 

 constructs its organic substance from the carbon dioxide which it can 

 decompose, but this process of assimilation only takes place in plants 

 that contain chlorophyll and in the light. Thus as 



many chlorophyll grains as possible require to be 

 exposed to the *light, and this is attained even 

 in massive bodies by flattened form. 



5. Dorsiventrality. The majority of the forms 

 so far referred to are radial or bilaterally sym- 

 metrical. In those in which the thallus spreads 

 out on a substratum (e.g. in many Lichens), the 

 construction of the plant body further becomes 

 dorsiventral. Dorsiventral symmetry is character- 

 istic of forms in which the upper side is the more 

 strongly illuminated and is especially concerned in 

 assimilation. 



6. Branching. Filamentous, ribbon -shaped, 

 and discoid forms, the surface of which is extended 

 as branches, are still more highly organised. This 

 occurs in most thalli of Algae, Fungi, and Bryo- 

 phyta. The free surface is still further increased 

 by the branching, and the available space is better 

 utilised. Thus bushy, shrub-like, and dendroid 

 thalli arise ; these in the Algae have often delicate 

 branches moving with the surrounding water to 

 which they offer little resistance. 



In branching the apex of the young plant may 



divide into two new and equivalent parts (DICHOTOMOUS BRANCHING), 

 as happens repeatedly in the fan-shaped thallus of the Brown Seaweed, 

 Didyota dichotoma (Fig. 83 ; cf. the diagram in Fig. 82 a). In other 

 branched forms there is a new formation of growing points which 

 give rise to lateral branches (LATERAL BRANCHING), and in the higher 

 forms this becomes more and more limited to the apical region of 

 the thallus ; the youngest and shortest lateral branches are the nearest 

 to the apex. Such an ACROPETAL origin of new lateral members is 

 already evident in the filamentous Green Alga, Cladopliora (Fig. 84 ; 

 cf. also Fig. 89). In the simplest case of lateral branching a single 

 main axis (MONOPODIUM) continues its apical growth throughout the 

 branch system. It behaves as the parent axis to a large number of 





FIG. 81. Ulva lactuca, 

 young stage, show- 

 ing apex and base. 

 ( x 220. After STRAS- 



BURGF.R.) 



