SECT. I MORPHOLOGY 163 



completely overgrown as to be concealed from view. The growing 

 points of adventitious shoots often arise in such masses of callus. As 

 the wood produced over wounds diflers in structure from normal 

 wood, it has been distinguished as CALLUS WOOD. It consists at 

 first of almost isodiametrical cells, which are, however, eventually 

 followed by more elongated cell forms. In the cherry instead of 

 normal wood elements there are produced on wounding the cambium 

 nests of thin-walled parenchymatous cells which undergo gummosis 

 (p. 109) (^^). At the base of cut-off portions of plants which are used 

 as cuttings a callus formation, which according to the kind of plant 

 is more or less active, takes place. In extreme cases the callus forms 

 projecting masses within which the rudiments of adventitious roots arise. 



All the tissues of the wounded surface so far as they contain living cells are in 

 this case also capable of forming callus, but the cambium is especially concerned in 

 it. The masses of callus formed at the two ends of a cuttinir with a cut surface 

 both above and below tend to exhibit differences in their behaviour. Thus in 

 such cuttings of species of Populus the apical callus develops more strongly than 

 the basal. In the basal callus a strand of meristem is protruded from the stem 

 cambium, which covers the exposed wood of the cutting with callus wood ; in the 

 apical callus this meristematic tissue forms a branched strand of tissue in which 

 vessels form. Under ordinary conditions only the basal callus is capable of 

 forming roots while the apical callus gives rise to new shoots Q^^). 



Regeneration. — Lost parts of the body of Fungi and Algae are 

 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 Q'^^). 

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

 been found to be replaced Q^^). As a rule, however, when re- 

 generation 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. 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 



In the section dealing with the development of form in the 

 vegetable kingdom (p. 10) the main aspects of the origin of the 

 external form of plants were considered. The phylogenetic 

 differentiation in the internal structure of a plant does not altogether 

 coincide with the progress of its external segmentation. Even 

 unicellular plants of the Siphoneae may exhibit a high degree of 

 external differentiation ; thus the unicellular Alga, Caulerpa (Fig. 

 286), has developed appendages having outwardly the forms of leaf, 



