56 SECONDARY INCREASE IN THICKNESS 



thing outside the cambium, but designates those tissues which 

 are cut ofi by the deep-lying cork layers as told above. 



In roots the phellogen takes its origin from the pericycle, 

 so that the whole of the primary cortex is shut off from the 

 interior water supply as soon as cork is formed and soon there- 

 after dies. It is, therefore, the periderm that constitutes most 

 of the bark of old roots. 



Monocotyledons 



Monocotyledons have no cambium ring and additions to the 

 vascular bundles cannot take place as in Dicotyledons. The 

 absence of a cambium ring is due to the fact that the procam- 



bium strands differentiate entirely 



into the permanent tissues of xylem 



and phloem, leaving none of their cells 



in the meristematic condition (Fig. 



28). Increase in thickness in most 



monocotyledons takes place simply by 



the enlargement of the cells of the 



permanent tissues that are formed 



from the primary meristems near 



Fig. 27.— Photomicrograph of the growing apcx, and this enlarge- 



comstair'whert thr^procaT ^ent, as a rule, soon ceases (compare 



bium strands have just gone Fjgg, 37 and 28); but in palms it con- 



over into vascular bundles. 



For compsrison with Fig. 28. tmucs for a long time in the ground 



tissue, including the sclerenchyma 

 sheath around the vascular bundles, until the diameter of the 

 stem has been doubled or trebled. This method of enlargement 

 does not increase the number of the vascular bundles, nor the 

 capacity of the food and water highways of those already exist- 

 ing, and the size of the crown of leaves which would evaporate 

 the water and supply the food cannot be permitted to increase 

 indefinitely from year to year, as in the case of Dicotyledons, 

 where the conducting highways are added to each year by the 

 cambium. In palms, for instance, the old leaves are shed about 

 as fast as new ones. are formed. 



