530 



BOTANY OF THE LIVING PLANT 



There is no need to describe the minute structure of the vegetative 

 organs, since it corresponds in essentials to what has been seen in 



Angiosperms. It must suffice 

 to note certain features of 

 comparative importance. The 

 stem is constructed on the 

 same plan as that of the woody 

 Dicotyledons, with indefinite 

 secondary thickening of the 

 vascular ring originating from 

 a cambium (compare Fig. 35, 

 p. 57). It results in the Scots 

 Pine in a woody trunk marked 

 by annual rings and medullary 

 rays, while externally are 

 phloem and a scaly fissured 

 bark, (see Fig. 42, p. 64). Resin- 

 passages permeate all the 

 tissues irregularly. They are 

 specialised intercellular spaces, 

 lined by an epithelium, which 

 deposits the sticky resin in the 

 passages. It exudes from them 

 under pressure of the surround- 

 ing tissues whenever the plant 

 is broken or cut. The most 

 notable structural feature is 

 that the wood is composed 

 entirely of tracheides, each 

 developed after tangential 

 1"%™! division from a single cambial 



= cam- 



flG. 417. 



Tracheides of Pine, seen in radial section 

 successive tracheides of one radial row. cb 

 bium. t = young pit of cambium, f, T=older pits. 11 /rp- . , ,-A Thpv arP lini- 

 5/ = pits of larger area facing the oblong cells of the cen \ ri fc>- 4 1 /,/* 1 "cy die uin 



medullary ray. ( x 550.) (After Sachs.) form in shape, as the cambium 



is, and are not deformed as the wood of the Dicotyledons is, by 

 sliding growth, or by unequal development of the individual cells. 

 The wood is consequently of that even texture seen in " deal." 

 The tracheides are lignified, and their radial walls are marked by 

 circular bordered pits. 



The bordered pit which is found widespread in the tracheae of vascular 

 plants, is seen in perfection in the wood of Conifers. The pit originates as a 

 circular area of wall which remains thin, while the rest of the wall thickens. 



