910 A TEXTBOOK OF THEORETICAL BOTANY 



There are undoubtedly many primitive characters in the structure of 

 trees. These have been attributed to the longer life of the tree and the 

 smaller number of generations which such species have passed through in 

 a given time, but nevertheless the balance of evidence is generally held to be 

 in favour of the arboreal theory. 



Two interesting anatomical theories have been propounded on the 

 supposed mode of origin of the herbaceous habit from the arboreal, which 

 may be thus briefly summarized : — 



1. Theory of Jeffrey. Where leaf traces enter the vascular cylinder there 



is a need for storage for the photosynthetic products of the leaf. 

 This is met firstly by a concentration of medullary rays flanking 

 the traces (aggregate rays). Secondly, these fuse into broad com- 

 pound rays and the neighbouring woody tissue becomes trans- 

 formed into storage parenchyma, so that the woody cylinder is 

 broken into separate wedges with broad rays between, through 

 which the leaf traces pass out. 



2. Theory of Sinnott and Bailey. The herbaceous habit may be attained 



in two ways, firstly and more simply by the reduction of cambial 

 activity, so that the stem is furnished with a continuous, but thin, 

 ring of wood which soon ceases to expand. Secondly by the 

 reduction of cambial activity in the sectors hehveen the leaf traces, 

 the wood there formed being first lessened and finally abolished, 

 leaving broad parenchyma tracts between the separate wedges of 

 wood to which the vascular ring is thus reduced. 



Detailed anatomical evidence can be produced for all these processes, 

 and it seems possible that each of them may have been operative in different 

 cases (Fig. 893). 



The anatomical features which we have discussed above have more 

 relation to the Dicotyledons than to the Monocotyledons, in which no process 

 of timber building occurs. Some Monocotyledons, however, such as the 

 Palms, form apparently woody trunks by a process which is quite special to 

 them, and which may be called difTuse thickening. In the early stages of 

 their development, the apical meristem broadens year by year, with very 

 little growth in length. On this broad meristem the number of leaf primordia 

 increases progressively, with a consequent increase in the number of trace 

 bundles in the stem. The shape of the stem is that of an inverted cone, 

 usually partly buried in the soil. When growth in length begins the stem 

 shoots up into a cylinder, no further growth in girth occurring. No cambium 

 is formed in the bundles, which remain permanently distinct. Each bundle 

 is surrounded or partly surrounded, from an early stage, by a belt of 

 sclerenchyma, and the only alteration of the tissues with age is the increased 

 thickening of the cell walls, which may in time involve the whole of the 

 ground tissue. A very solid stem may thus be formed, even though it contains 

 no mass of wood, and although the parenchyma cells of the ground tissue may 

 remain alive and are often filled with stored starch grains. 



