56 SECONDARY INCREASE IN THICKNESS 



thing outside the cambium, but designates those tissues which 

 are cut off 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 apex, and this enlarge- 



cross section of very young i 



cornstalk, where the procam- men t, aS a rule, SOOn CCaSCS (compare 



bium strands have just gone FigS. 27 and 28); bllt in palms it COn- 

 over into vascular bundles. . . 



For comparison with Fig. 28. tinues 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. 



