152 



BOTANY 



often replaced, a direct regeneration thus taking place. In more 

 highly organised plants such a proceeding is extremely rare. It 

 occurs most readily in embryonic organs, such as growing points, when 

 portions have been lost, and is most often found in seedlings. Thus 

 in seedling plants of Cyclamen even a severed leaf-blade has been 

 found to be replaced. As a rule, however, when regeneration 

 processes are requisite in higher plants and the necessary preformed 

 organs are not present in a resting or latent condition, the older 

 tissues return to the embryonic condition and give rise to new 

 growing points of shoots ( 143 ). That this is a provision for the 

 indirect replacement of lost parts is confirmed by the fact that direct 

 regeneration is far more frequent among animals than plants. 



The Phylogeny of the Internal Structure 



The phylogenetic differentiation in the internal structure of a 

 plant does not altogether coincide with the progress of its external 



Fir,. 160. Surface view and transverse section of the thallus of Marchantiapolymorpha. In A, an 

 air-pore, as seen from above ; in B, as seen in cross-section, (x 240.) 



segmentation. Even unicellular plants of the Siphoneae may 

 exhibit a high degree of external differentiation ; thus the uni- 

 cellular Alga, Caulerpa (Fig. 295), has developed appendages 

 having outwardly the forms of leaf, stem, and root. Similarly, the 

 Red Seaweed, Hydrolapaflium (Fig. 9), although composed almost 

 entirely of one form of cells, bears in its external segmentation a 

 resemblance to one of the most highly organised plants. The 

 internal differentiation of this Alga has only advanced so far, that 

 the outer cells containing the red chloroplasts form an assimilat- 

 ing tissue of isodiametrical cells, while the internal colourless and 



