NON-ADAPTIVE ANOMALIES 699 



secondary phloem externally and secondary xyleni internally in per- 

 fectly normal fashion, are rarely complete ; as a rule each only extends 

 over a portion of the circumference, its edges intersecting the next 

 older cylinder or curved strip of cambial tissue. According to Morot, 

 each new arc of cambium arises opposite a phloem group, and extends 

 laterally on either side until it meets an older cambial layer. Herail, 

 on the contrary, maintains that each new cambium begins to develop 

 at one end as a lateral continuation of an antecedent cambial layer ; 

 thence it extends gradually across the leptome, and sooner or later 

 rejoins an older cambial strip. Leisering believes that both possi- 

 bilities may be realised. The individual strips of cambium are often 

 small and very numerous, in which case they present the appearance 

 of a dense network in a transverse section ; since every part of this 

 meristematic reticulum continually produces secondary xylem on its 

 inside and secondary phloem on its outside, the final product of its 

 activity is a homogeneous woody cylinder comprising vessels and xylem- 

 parenchyma, and usually also numerous wood-fibres which encloses 

 isolated leptome-strands. According to Leisering, this type of secon- 

 dary thickening occurs not only in the above-mentioned families, but 

 also among the Hippocrateaceae, Plumb agin aceae, Melastomaceae, 



LoRANTHACEAE, COMBRETACEAE, etc. 



Some account must, finally, be given of the secondary growth in 

 thickness of Cycad-trunks. 3 '''' Among Cycads the arrangement of the 

 vascular tissues deviates considerably from the normal type, even in 

 the primary condition of the stem. The broad leaf-bases contain 

 numerous bundles : at the surface of insertion these are reduced by 

 fusion to a single pair of leaf-trace strands, which at first follow a 

 peripheral and nearly horizontal (tangential) course in the stem, but 

 finally run obliquely downwards and inwards to join the circle of 

 primary bundles situated between pith and cortex. Both these last- 

 mentioned tissues are bulky, and consist of thin-walled parenchyma, 

 which contains a large amount of starch, and in all probability mainly 

 serves as a storage-tissue. The two members of each pair of leaf-traces 

 are linked together by a horizontal commissure, the whole structure 

 forming a transverse ring or girdle ; further anastomoses appear, some 

 of which connect neighbouring girdles with one another, while others 

 place the girdles in direct communication with the primary bundles. 

 In this way a richly branched cortical vascular reticulum is formed, a 

 feature which is no doubt correlated with the great thickness of the 

 cortical region. All these original vascular bundles collectively 

 termed by De Bary the "primary network of bundles" are collateral 

 in structure. 



Secondary growth in thickness is initiated by the appearance, in 



