68 THE TISSUES OF PLANTS 



. Fig. 17, is a specimen of adipose tissue from a female 

 seventy years of age. ' Eaci cell contains a stellate mass 

 of needle-shaped crystals.* 



The vasiform tissue and ducts in early spring, when the 

 ascent of the sap is most powerful, at first convey it to the 

 leaves in conjunction with the fibre-cells of the wood. But 

 as the flow of the sap becomes less vigorous, it gradually 

 disappears from the ducts and spiral vessels, owing to their 

 deficiency in the requisite amount of capillarity, which thus 

 become filled with air. In this second period of vegetation 

 they become organs of respiration, and as they are spread 

 through the interior of the stem and enter the leaves through 

 their foot'stalks, communicating with the intercellular spaces 

 amongst the cells of their parenchyma, and with the pores 

 on their outer surface ; the sap in the interior of the plant 

 is thus brought, through their agency, into constant commu- 

 nication with the nutritious gases of the atmosphere. 



As the sap very soon retires from the vasiform tissue 

 and ducts, the earthy deposits on their parieties do not tale 

 ptace to any very great extent, and hence they retain their 

 tubular character throughout the life of the plant. As 

 the force of the ascending current -diminishes, it forsakes 

 the large capillaries of the fibro-vascular tissue; for the same 

 reason, its flow through the finer capillary tubes of the fibre- 

 cells continues, after its flow in the fibro-vascular system 

 ceases. Hence earthy matter goes on accumulating in the- 

 fibre-cells, until it ultimat&ly fills up their cavities, and ob- 

 literates their tubular character. When ,this is the case, 

 their vital activity ceases, and they exercise a purely 

 mechanical function. To the deposits of earthy matter in 

 the fibre cells of the wood, the term lignine is applied; in 



* "Lectures on Histology, Ac.," by Jolin Quekett. p. 187. 



