DORSIVENTRAL SHOOTS. POSITION OF ORGANS 



without direct relationship to the outer world. Some marine Algae in 

 which we frequently can see but little relation to the environment may be 

 examples of this. Thus Rytiphloea pinastroides x possesses non-creeping 

 shoots which are free in the water and have dorsiventral involute termina- 

 tions like croziers. The lateral shoots stand upon the flanks towards 

 the concave side ; short shoots (' leaves ') of simple construction stand upon 

 the convex side. The former, along with the involution, protect the 

 vegetative point, but I must leave as an open question whether this is the 

 only biological significance of the dorsiventrality. Cliftonaea pectinata, 

 the alga represented 

 in Fig. 40, has one 

 side developed as a 

 wing which is evi- 

 dently vertical in 

 profile. 



A remarkable 

 parallel formation is 



FlG. 42. Thuya occidentalls. 

 Scheme of the position of the 

 twigs on a lateral shoot. The 

 first branches of the third order 

 stand on that side of the branches 

 of the second order which is 

 turned towards the chief axis. 



FlG. 43. Antithamnion (Pterothamnion) Plumula. Portion of the thallus. 

 The small cells with peculiar contents are not shown. Highly magnified. 

 The explanation of the figure will be found in the text. 



seen in the liverwort Riella (Fig. 41). The shoots here are also dorsi- 

 ventral, but the dorsal side is not flat as is the case in other dorsiventral 

 liverworts, it is developed as a wing when seen in profile a . We find 

 similar arrangements in leaves, for example the leaf-surface in Fissidens 

 and Iris corresponds with the wing in Cliftonaea and Riella. 



1 See for this and some other analogous cases Ambronn, Uber einige Falle von Bilateralitat bei 

 den Florideen, in Botan. Zeitung, 1880, p. 160. 



2 For the details see Part II of this book. 



