IS8 



THE STEM PROPER 



303. — Transverse section of a stem of bur- 

 dock, showing fibrovascular bundles not 

 completely united into a ring. 



■w{Fig. 302), intermingled with larger sized tubes or ducts, 

 V, the sections of which made the pores referred to in 

 Section 221. In front of these is the cambium layer, c, and 



beyond that, the soft bast 

 and other tissues in which 

 elaborated food is being 

 brought down from the 

 leaves and material for 

 growth provided. In very 

 young stems the vascular 

 bundles are separate and 

 distinct, as in Figure 303, 

 being connected only by 

 a ring of cambium, but 

 as growth advances and 

 more bundles are formed 

 to supply the new buds 

 and leaves of the devel- 

 oping axis, they become crowded into a ring (Fig. 304), 

 which is separated into woody wedges by the threads of 

 pith (medullary rays) that run between them from the 

 center to the cortex. The cambium constantly advances 

 outwards, beginning every spring a new season's growth 

 and leaving behind the ring of ducts 

 and woody fibers made the year before. 

 As the work of the plant is most active 

 and its growth most vigorous in spring, 

 the largest ducts are formed then, the 

 tissue becoming closer and finer as 

 the season advances, thus causing the 

 division into annual rings that is so 

 characteristic of dicotyl stems. Each 

 new stratum of growth is made up of 

 the fibrovascular bundles that supply 

 the leaves and buds and branches of the season. Fig- 

 ure 305 gives a diagrammatic section illustrating the 

 passage of the bundles from the leaves to the stem of a 

 dicotyledon, each successive node sending down its quota. 



304. — Diagram of an 

 older dicotyl stem, show- 

 ing bundles confluent into 

 a ring (GRAY). 



