SIZE VAUIATIOX IN TUACIIKAUV CELLS. 181 



The most salient features of these tables are the following: (1) 

 There is a marked contrast in length between the innermost or first 

 formed and subsequently formed tracheary elements of the secondary 

 xylem. In Ginkgo and the Coniferae, the latter are from 200-600 

 percent longer than the former. There is a similar, though less pro- 

 nounced, contrast in size between the innermost and outermost 

 tracheids, fiber-tracheids and libriform fibers of the secondary xylem 

 of dicotyledons. (2) The vessel-segments of the dicotyledons are 

 invariably somewhat shorter than the surrounding tracheary elements 

 of the secondary xylem; but in certain stems, the A'essel-segments of 

 the first formed growth layers are of the same length as those which 

 occur in the later formed wood. (3) Although extremely variable in 

 size, the tracheids of Gymnosperms are, on an average, noticeably 

 longer than equi^•alent tracheary elements of dicotyledons, Table III. 

 This contrast in size is somewhat less conspicuous in the case of the 

 tracheary elements of the first formed secondary xylem. (4) In the 

 dicotyledons, the fiber-like tracheary elements tend to become shorter 

 as the vessel-segments decrease in length. Table III. (5) In the 

 dicotyledons, there is no close correlation between the size of the 

 tracheary elements and the various types of floral organization which 

 form the basis of the Engler and Gilg (1912) system of classification. 

 (6) In the Coniferae, the tracheary elements of the Taxaceae and 

 Cupresseae average somewhat shorter, and those of the Taxodieae 

 and Araucarieae somewhat longer, than the tracheids of the Abieteae, 

 Table III. 



Sanio (1872) came to the conclusion that in the stems and branches 

 of Pinu3 sylvcstris Linn, the tracheids everywhere increase from wdthin 

 outwards, throughout a number of annual rings, until they have at- 

 tained a definite size, which then remains constant for the following 

 annual rings. Shepard and Bailey (1914) and subsequently Miss 

 Gerry (1915) and Lee and Smith (1916) have shown that Sanio's law 

 is not applicable to conifers in general. In Pinus Strobus Linn., P. 

 palustris Mill., Picea rubens Sarg., Pseudotsuga taxifolia Britton, 

 Tsuga canadensis Carr., and Abies concolor Lindl. and Gord., the 

 tracheids were found to increase rapidly in length for a period of years, 

 varying from 20-60; but no constant length was attained even in 

 trees several hundred years old. Following the first period of rapid 

 increase in length the size of the tracheids fluctuated more or less 

 during the subsequent growth of the plant. Similar results have been 



5 Sanio's constants occurred after 18-40 years. 



