78 ORGANOGRAPHY. 



Stems are called exogenous. In the stems of Coniferous Plants, 

 as those of the Fir, Yew, Cypress, &e., the annual zones, which 

 are well marked (Jig.\72), consist chiefly of punctated woody 

 tissue (see p. 13 and 30), with occasionally a few pitted vessels 

 intermixed. 



The pitted vessels, which we have seen form a portion of each 

 annual layer of the wood, arc so large in the Oak, Ash, &c., that 

 they may readily be seen by the naked eye upon making a trans- 

 verse section of such trees. Upon examining under the micro- 

 scope a transverse slice of any common exogenous stem, the 

 pitted vessels may be at once distinguished from the wood- 

 cells by the larger size of their openings (Jig. 169, v). In the 

 ConifeViE, where but few if any pitted ducts occur, a trans- 

 verse section shows the orifices nearly all of equal size, with 

 occasionally a pitted vessel intermixed (Jig. 172). These pit- 

 Fig. 172. 



Fig. 172. Horizontal section of the stem of a Fir three years old. The 

 figures 1, 2, o, refer to the annual layers of wood. la. Cavities contain- 

 ing resinous secretions. 



ted vessels in ordinary trees are also commonly more abun- 

 dant on the inner part of each annual zone, the Avood-cclls 

 forming a compact layer on the outside. In such cases the 

 limits of each zone arc accurately defined. In those trees which 

 liave the pitted vessels more or less diifuscd throughout the 

 woody tissue, as in the Lime, Maple, &c., the zones are by no 

 means so evident, and can then only be distinguislied by the 

 smaller size of the wood-cells on the outside of each layer, which 

 api)carance is caused by their dimini.^licd growth towards the 

 end of the season. The distinction l)etwcen tlie annual zones is 

 always most evident in trees growing in temperate and cold 

 climates, where there is a more or less lengtlicncd winter in 

 wiiicli no growth takes ])lace, followed by rapid vegetation after- 

 wards. In the trees of tropical climates the zones are not so 

 clearly defined, because there is no comjilcte season of repose in 

 such regions, although tu a certain extent the dry season here 

 leads to a cessation o}"growtli, but the alternation of the growing 



