332 



BOTANY 



diameter in Sequoia of ten metres or more, near the base (PL I, 

 frontispiece). The leaves are always small, and often needlelike, 



differing very much in this 

 respect from the Cycads. 

 In Agathis and Araucaria, 

 the leaves are broader than 

 in the other genera ; and in 

 Sciadopitys, the leaves are 

 rudimentary and replaced 

 by flattened branches, or 

 phylloclades. 



The stem branches freely, 

 the branching being usually 

 very symmetrical, so that 

 the trees often assume a 

 perfect conical shape. This 

 is mainly due to the per- 

 sistence of the apical bud, 

 which results in a straight 

 central shaft about which 

 the lateral branches are 

 regularly disposed. 



A tap-root is present in 

 the young sporophyte, and 

 may persist for a long time, but is often replaced by secondary roots. 

 Distribution. The Coniferae are cosmopolitan, but are best de- 

 veloped in the temperate regions of the northern hemisphere. They 

 reach their maximum development in the countries about the 

 northern Pacific, both in Asia and America. The forest of our own 

 Pacific slope is composed in great part of coniferous trees, which 

 here reach their greatest size. 



FIG. 294. Ginkgo biloba. Fruit and seed. 

 (Natural size.) (After BAILEY.) 



The Stem 



The apex of the stem in the Coniferse probably never grows from a single initial 

 cell. A group of terminal initials, much like that in the Cycadaceae and Lyco- 

 podium, has been demonstrated in the Abietinese. In other forms, e.g. Arau- 

 caria, Cunninghamia, Sequoia (Fig. 295), a single group of initials is not present, 

 but the primary tissue-systems are continuous over the apex of the shoot. The 

 central tissue-cylinder, from which is derived the pith, can be easily followed to 

 the apex. Overlying this is the periblem, from which the cortex and vascular 

 bundles arise, and the dermatogen, or primary epidermis, forms a single layer 

 over the apex. 



The leaves arise as lateral outgrowths of the stem, and show much the same 

 distribution of their young tissues. A single leaf-trace, or vascular bundle, 

 passes from each leaf into the stem, and the unions of these leaf-traces form the 



