THE ANGIOSPERMAE : STEMS 891 



ment is indeed in a somewhat hazy state. For example, although basipetal 

 differentiation of vascular elements within the procambium seems to be the 

 rule, it is by no means certain that the differentiation of the procambium 

 itself, from the primary meristem, is not acropetal or continuous. The 

 above account must therefore be regarded as provisional. 



The development of the meristematic cells in a young leaf axil into an 

 axillary bud complicates the dicolytedonous nodal structure further by its 

 special system of traces. The axillary bud has an apex which is a miniature 

 of the main apex and has the same procambial arrangement, giving rise to a 

 duplicate of the stem stele. As this differentiates downwards into the stem, 

 the ring or group of traces opens out fanwise, forming two bundles which pass 

 into the main ring between the bundles which flank the median leaf gap and 

 join them above the level at which the median trace of the subtending leaf itself 

 joins an older trace in the ring. 



Exogenous adventitious buds such as those on the corms of Cyclamen 

 are apparently connected secondarily to the stele of the parent stem. Pro- 

 cambial strands come from the bud and differentiate inwards through the 

 cortex towards the stele, and the formation of vascular tissue in this pro- 

 cambium is likewise downwards from the bud. 



7. The Cambium. It has been previously pointed out that the organization 

 of cambium from procambium marks the beginning of radial growth, 

 commonly called secondary growth, which is capable of indefinite expansion 

 outwards and will, if continued, build up a massive woody stem. 



In many herbaceous stems, although secondary growth begins as in 

 woody plants, it does not continue beyond one year. The structure of such 

 herbaceous stems is therefore similar to that in the one-year-old stems of 

 woody plants. 



The cambial initials are distinguished in the procambium by their repeated 

 tangential, longitudinal divisions, giving rise to a zone of cells which appear 

 narrow in transverse section. This occurs in the part of the stem apex where 

 active growth is still going on, and the cambial initials undergo considerable 

 elongation and become radially compressed by the expanding tissues of the 

 pith and cortex. In spite of these changes the cambial cell remains a true 

 meristematic cell, and its repeated tangential divisions cut off new xylem 

 cells centripetally and new phloem cells centrifugally. It is difficult to 

 distinguish these new tissue elements in their early stages of development 

 from the true cambial cells, and it is not possible to say in every case whether 

 the fully developed cambium consists of only one layer of cells or of more. 



The position at which the cambium forms in the procambium may be 

 physically determined by the gradient between acid sap from the differentiating 

 wood and alkaline sap from the phloem. This theory, which is due to 

 Priestley, maintains that the cambial layer lies at the p\l on the gradient 

 which corresponds to the mean isoelectric point of the cell proteins (about 

 pH 4-5), and that the protoplasm of the cambial cells remains unvacuolated 

 because at the isoelectric point colloids have their minimum affinity for water 

 and their maximum density. 



