130 



BOTANY 



I' ART I 



The tracheal water-courses in the wood of Dicotyledons are more 

 or less completely isolated from each other. Their isolation is the more 

 complete where, as in the Leguminosae, Willow, and Ficus, vessels are 

 the only water-carriers. If both tracheae and tracheids are present, 

 as in the Lime, then the tracheids unite the tracheae together and the 

 conduction of water is rendered possible in all directions. In any 

 case, a union exists at the junction of the annual rings, between the 



Fig. 145. — Transverse section of a stem of Tilia 

 parvifolia, in the fourth year of its growth. 

 pr, Primary cortex; c, cambium ring; cr, bast; 

 pm, primary medullary ray; pm', expanded ex- 

 tremity of a primary medullary ray ; sm, second- 

 ary medullary ray ; g, limit of third year's wood. 

 (X6.) 



Fig. 144.— Elements of the 

 parenchymatous tissue of 

 tho wood ; diagrammatic. 

 (For description, see text.) 



tracheal tissues of successive years. Large vessels are characteristic of 

 climbing woody plants (Lianes, Fig. 151), but they are accompanied 

 by smaller ones, with which they are in communication. When 

 both large and small vessels are present together, the smaller appear 

 to act as water-carriers, while the larger are utilised as water-reservoirs. 

 Whenever communication takes place between tracheal elements, and 

 it always occurs when they are in direct contact, it is effected by 

 means of bordered pits or actual openings. The distribution of the 



