128 



BOTANY 



PAKT I 



While the tracheal tissues are engaged in providing for the conduc- 

 tion of water, the duty of conducting and storing the products of 

 assimilation, in particular the carbohydrates, is performed by the 

 parenchymatous tissues of the wood. Both forms of tissue, however, 

 aid in maintaining the rigidity and elasticity of the plant body, and, in 

 their most extreme development, furnish such elements as the fibre 

 tracheids on the one hand, and on the other the empty wood fibres, 

 which are only capable of performing mechanical functions. 



The wood of Dicotyledons is made up of the elements of these 





Fig. 141. — Radial section of a Pine stem, at the junction of the wood and bast, s, Late 

 tracheids ; t, bordered pits ; c, cambium ; v, sieve-tubes ; vt, sieve-pits ; tm, traeheidal medul- 

 lary ray cells ; sm, medullary ray cells in the wood, containing starch ; sm' t the same, in the 

 bast; em, medullary ray cells, with albuminous contents, (x 240.) 



two classes of tissue, the tracheal and the parenchymatous, but all the 

 different elements are not necessarily represented in any one kind 

 of wood. 



i. Drimys, a genus closely allied to the Magnolias, is the only Dicotyledon of 

 which the wood is formed solely of tracheids. This Dicotyledon closely resembles 

 the Conifers in structure. In numerous Leguminosae, Willows, Poplars, and species 

 of Ficus, on the other hand, the tracheal tissues are only represented by vessels, 

 which perform the task of water-conduction. In the wood strands of these plants 

 there are also present wood parenchyma and a large amount of wood fibres which 

 contain only air. In Maples, on the contrary, the wood fibres contain living proto- 

 plasm and starch ; this circumstance renders the formation of wood parenchyma 



