THE ANGIOSPERMAE : STEMS 909 



The above characters are grouped as speciaHzed {i.e., advanced in the 

 evolutionary sense) or unspeciaUzed, as follows : — - 



Unspecialized. Specialized. 



Scalariform vessel pores. Simple vessel pores. 



Fibre tracheids. Libriform fibres. 



Apotracheal parenchyma. Paratracheal parenchyma. 



Diffuse-porous. Ring-porous. 



Unstoried structure. Storied structure. 



Specialized and unspecialized features are widely distributed among 

 angiospermic families and show no relation to specialized or unspecialized 

 floral structure, but truly natural families are usually fairly homogeneous in 

 their type of wood structure. 



12. The Pith. The comparative anatomy of pith is still to be written, 

 though it is evidently a very variable tissue. Many herbaceous plants, notably 

 among the Umbelliferae and Compositae, have hollow stems, due to the 

 failure of the pith to keep pace with the expansion of the outer tissues. The 

 pith thus becomes cracked and riven, or fistular as it is called, only a few 

 scanty fragments of tissue remaining to line the cavity. Certain woody plants, 

 of which the Walnut {Juglans regia) is the best known, have a discoid pith, 

 due to transverse cleavage, which, consequent upon growth in length of the 

 young shoot, separates the pith into a series of discs, 2 to 3 mm. apart. 



The pith often becomes sclerotic, especially at the nodes, and nodal 

 plates of highly thickened cells may remain even when the pith has dis- 

 appeared in the internodes. The outer zones of the pith are frequently 

 smaller celled, more sclerotic and more persistent than the central portion, 

 forming a conspicuous boundary layer, called in German " Markrande," 

 though an international name has not been adopted. 



The Arboreal and Herbaceous Habits 



There has been some controversy on the question whether the arboreal 

 or the herbaceous habit of growth is primitive in Angiosperms. Arguments 

 may be adduced on both sides. In favour of the arboreal habit there is the 

 fact that the earliest known fossil Angiosperms appear to belong almost 

 exclusively to tree families. Further there is the present predominance of 

 woody forms in tropical climates, together with the parallels which can be 

 drawn between woody tropical species and related herbaceous species in 

 temperate climates ; the argument being that the herbaceous habit is an 

 adaptation to the severity of winter. On the other side is urged the fact 

 that the youngest stages of tree species are herbaceous, which is an appeal 

 to the theory of recapitulation (see Volume HI). Moreover a parallel 

 can be drawn between the great stature of trees with the large mass of their 

 non-living tissue and the gigantism and heaw bone-structure of decadent 

 animal groups of the past, such as the Mesozoic reptiles. 



