Differentiation 201 



process of growth but is not part of the meristem proper and which he 

 terms a meristemoid, inhibits the development of others near it. 



The differentiation of tissues produced by the vascular cambium has 

 been studied by Linnemann (1953), who observed that in beech the 

 proportion of rays is greater in the wood of isolated trees than in dense 

 stands but that it does not vary consistently with age or as between 

 trunk and branches. Rays tend to increase in width during an annual 

 ring and to be wider in the narrower rings. 



The fact that vessel elements occur in longitudinal series to form a 

 duct indicates the operation of a continuous stimulus longitudinally 

 along the axis. Priestley, Scott, and Malins (1935) have shown that a 

 single duct differentiates almost simultaneously throughout a long extent 

 of trunk. A leaf trace passing down into a young stem exerts a consider- 

 able correlative influence upon vessel differentiation below it. Alexandrov 

 and Abessadze (1934) found that there are fewer vessels in a segment 

 just below a leaf trace and that they appear earliest next the rays that de- 

 limit the trace. The vessel-forming stimulus clearly moves downward, 

 thus suggesting the operation of a hormonal control. 



That auxin has a role in the initiation of the ring-porous condition is 

 suggested by the work of Wareing (1951), and Chowdhury (1953) has 

 analyzed some of the factors responsible for the transformation of 

 diffuse-porous to ring-porous structure in Gmelina. 



Continuity in differentiation of similar cells is also shown by the cork 

 cambium, or phellogen (p. 88), in old cortex or phloem. Here it arises 

 as a series of almost simultaneous divisions which, as seen in transverse 

 section, somewhat resemble those described for the Luffa strands, since 

 they are connected to one another in a series and often follow a somewhat 

 irregular course through the tissue in which they arise. They form a 

 continuous sheet of meristematic cells, often in localized patches, which 

 cut off elements from their outer faces. Their cells suberize later, thus 

 sealing off the outside tissues. The origin of these phellogens is the more 

 interesting because they have their beginning in tissues where the cells 

 are mature and intermixed with dead or necrotic ones. Their origin 

 after wounding is related to the operation of wound hormones. 



A notable example of the differentiation of a histological pattern is 

 furnished by the system of lignified thickenings (the "reseau de soutien" 

 of van Tieghem, 1888) in the air roots of certain orchids, which is pre- 

 sumably concerned with providing rigidity for tissue otherwise soft and 

 easily collapsed. These arise as bands of lignified wall thickenings which 

 surround individual cells. They may occasionally fork. It is noteworthy 

 that the band in a given cell is directly contiguous to that in an adjacent 

 cell, so that a continuous patterned network of thickened strands is 

 established (Fig. 8-14). This reminds one of the way in which the ringed 



