138 BOTANY. 



the stein. If we examine the inner side of the ring, the change the cells 

 undergo is more marked. They become of nearly equal diameter in all 

 directions, and the walls become woody, showing at the same time distinct 

 stratification (J5, x). 



On examining the xylem, where two growth rings are in contact, the 

 reason of the sharply marked line seen when the stem is examined with 

 the naked eye is obvious. On the inner side of this line (/), the wood 

 cells are comparatively small and much flattened, while the walls are 

 quite as heavy as those of the much larger cells (//) lying on the outer 

 side of the line. The small cells show the point where growth ceased at 

 the end of the season, the cells becoming smaller as growth was feebler. 

 The following year when growth commenced again, the first wood cells 

 formed by the cambium were much larger, as growth is most vigorous at 

 this time, and the wood formed of these larger cells is softer and lighter 

 colored than that formed of the smaller cells of the autumn growth. 



The wood is mainly composed of tracheids, there being no vessels 

 formed except the first year. These tracheids are characterized by the 

 presence of peculiar pits upon their walls, best seen when thin longi- 

 tudinal sections are made in a radial direction. These pits (Fig. 70, 

 Z>, p) appear in this view as double circles, but if cut across, as often 

 happens in a cross-section of the stem, or in a longitudinal section at 

 right angles to the radius (tangential), they are seen to be in shape some- 

 thing like an inverted saucer with a hole through the bottom. They are 

 formed in pairs, one on each side of the wall of adjacent tracheids, and 

 are separated by a very delicate membrane (F, p, 6r, y). These "bor- 

 dered" pits are very characteristic of the wood of all conifers. 



The structure of the root is best studied in the seedling plant, or in a 

 rootlet of an older one. The general plan of the root is much like that 

 of the pteridophytes. The fibro-vascular bundle (Fig. 75, If, /6.) is of 

 the so-called radial type, there being three xylem masses (x) alternating 

 with as many phloem masses (ph) in the root of the seedling. This 

 regularity becomes destroyed as the root grows older by the formation of 

 a cambium ring, something like that in the stem. 



The development of the sporangia is on the whole much like that of 

 the club mosses, and will not be examined here in detail. The microspores 

 (pollen spores) are formed in groups of four in precisely the same way as 

 the spores of the bryophytes and pteridophytes, and by collecting the male 

 flowers as they begin to appear in the spring, and crushing the sporangia 

 in water, the process of division may be seen. For more careful exami- 

 nation they may be crushed in a mixture of water and acetic acid, to 

 which is added a little gentian violet. This mixture fixes and stains the 



