THE ANGIOSPERMAE : STEMS 863 



Xylem consists of tracheids, which are individual cells, and of vessels, 

 which are files of cells opening into one another longitudinally to form long, 

 open tubes. The walls of these cells are lignified in various patterns, usually 

 spiral or annular in the earliest part formed, known as protoxylem, which 

 lies next to the pith ; and reticulate or pitted in the later-formed tissue, called 

 the metaxylem. There are also present xylem parenchyma cells with 

 lignified walls and fibre cells. Of all these xylem cells only the parenchyma 

 have any living contents. 



Phloem consists of sieve tubes, which are united longitudinally through 

 perforated end walls called sieve plates, through which the cytoplasm is 

 continuous. Sieve tubes have degenerate cytoplasm, but no nuclei, and are 

 short-lived. Their walls are formed of thick, soft cellulose. They are accom- 

 panied by long, narrow companion cells, and often by phloem parenchyma 

 cells. 



In Dicotyledons the number of trace bundles to each leaf is usuallv small 

 and they form a single ring in the internodes, separating the pith inside 

 from the cortex outside (Fig. 848). The cambium is continued across between 

 the bundles as the interfascicular cambium, from which develop secondary 

 tissues on both sides. If these consist only of parenchyma cells the bundles 

 of the ring remain separated by broad parenchymatous primary medullary 

 rays (Fig. 849). If the interfascicular cambium forms xylem and phloem 

 tissues, these ser\'e to unite the vascular bundles into continuous rings, 

 which are added to annually so long as the plant lives, and in time may 

 build up the masses of w'ood which are characteristic of timber trees. Each 

 yearly increment is distinguishable in the wood as an annual ring (Fig. 

 850). Similar annual rings are added to the phloem, but are much narrower 

 and seldom visibly distinct. Narrow secondary medullary rays are also 

 formed in the secondary vascular tissues and persist through their whole 

 thickness (Fig. 851). In small herbaceous plants there may be no cambium. 

 In Monocotyledons the leaves usually have broad bases and many leaf- 

 trace bundles. The stem is crowded with bundles, which do not lie in a 

 regular ring (Fig. 852). As a rule the leaves are set close together, and inter- 

 nodes may be very short or non-existent. Only in a few- rare cases is there 

 any cambium or any secondary growth in a Monocotyledon. 



Outside the ring of bundles in a Dicotyledon there may or may not be a 

 visible endodermis, but there is usually a pericycle, consisting chiefly of 

 woody fibres. In Monocotyledons both these may be absent. 



Outside these layers lies the parenchymatous cortex, bounded externally 

 by the epidermis. When secondarv' growth has commenced a new cambium, 

 called the phellogen or cork cambium, appears, variably located somewhere 

 between the limits of the phloem and the epidermis in different species 

 (Fig. 853). Its cells produce phellem or cork outwardly, and often also 

 phelloderm or secondary cortex inwardly. The cork cells are suberized 

 and impervious to water. Thev are, therefore, dead cells and form an imper\-i- 

 ous coating to the stem, which replaces the earlier epidermis. The whole 

 phellogen system is collectively termed the periderm. 



