158 BOTANY TART i 



HEART-WOOD. Only the outer layer of the wood composed of the 

 more recently -formed annual rings thus contains living cells and 

 constitutes the SPLINT-WOOD. Eeserve materials can only be stored 

 in the splint-wood, and water-conduction is also limited to this, and 

 indeed to its outermost portion, since, as has been seen, it is only the 

 peripheral vessels that are in connection with the leaves and the 

 youngest lateral roots. The heart-wood serves only for strength. 

 Less commonly the whole of the wood persists as splint-wood (species 

 of Maple, Birch). The heart-wood is usually darker in colour than 

 the splint-wood and is also denser, harder, and stronger \ it is protected 

 against decay by impregnation with various substances. In other 

 cases the heart- wood is not distinct in colour from the splint-wood 

 and readily decays ; this leads to the hollow stems so often found in 

 old Willows. 



The whitish yellow splint-wood contrasts most strongly with the heart-wood 

 when the latter is dark in colour ; thus in the Oak it is brown and in the Ebony 

 (Diospyros) black. The heart-wood appears to be more durable the darker it is. 

 Before their death the living cells of the wood, which lose their reserve materials, 

 usually form various organic substances, especially tannins, which impregnate the 

 walls of the surrounding elements, while resinous and gum-like products accumu- 

 late in the cavities. The tannins preserve the dead wood from decay, and their 

 oxidation products give its dark colour. The vessels are sometimes occluded by 

 accumulations of gum, and at other times by cells which fill up the lumen more 

 or less completely, and are spoken of as THYLOSES ( 80 ) (Fig. 183) ; they originate 

 by the adjoining living cells growing into the vessels through the pits, the 

 membrane of which they press inwards. Thyloses also form in wounded vessels 

 and occlude the lumen. Inorganic substances are not uncommonly deposited in 

 the heart-wood ; thus calcium carbonate occurs in the vessels of Ulmus campestris 

 and Fagus sylvatica, while amorphous silicic acid is deposited in the vessels of 

 Teak (Tectona grandis). Colouring matters are obtained from the heart- wood 

 of some trees, e.g. Haematoxylin from Haematoxylon campechianum L. (Campeachy- 

 wood, Logwood). 



4. The Bast. A. Kinds of Tissue and their Functions. 



Three types of tissue can also be distinguished in the bast (Figs. 175 

 B, 184): (1) Longitudinally-running strands of SIEVE-TUBES (v) with, 

 in the Dicotyledons, COMPANION CELLS (c) ; (2) in many plants longi- 

 tudinal strands of SCLERENCHYMATOUS FIBRES (BAST FIBRES) that are 

 as a rule dead (Fig. 184 /) ; and (3) PARENCHYMA with intercellular 

 spaces arranged both longitudinally (p) and in the medullary rays 

 (Figs. 175 B, m; 184 r). In addition SECRETORY CELLS of various 

 kinds may be present containing crystals (k) or latex. The bast, like 

 the phloem of the vascular bundles, serves mainly to conduct the 

 products of assimilation. It also is of use for the storage of organic 

 substances and frequently as a mechanical tissue. In many plants 

 the sieve-tubes have oblique end-walls (Fig. 184 v*) ; they are thin- 

 walled and unlignified, contain proteids, and usually remain functional 

 only for a short period. The bast fibres are long and narrow and 



