CELLULAR TISSUE. 



39 



may be as certainly identified by the minute sculpture of its cells 

 alone, as by more conspicuous external characters. They are 

 preserved even when the tissue is fossilized, and the external form, 

 with every outward appearance of organization, is obliterated. 

 Through thin slices and other contrivances, the hidden structure 

 is revealed under the microscope, and thus the true nature of our 

 earth's earliest vegetation may be often satisfactorily made out.* 

 The simplest cases of these markings are those of 



44. Dots or Pits, often 

 taken for pores, such 

 as those on the cells of 

 the pith of Elder (Fig. 

 25), and upon those 

 that are called dotted 

 ducts; as in Fig. 39, 

 and Fig. 21, I. All 

 markings of this kind 

 are thin spots, which, 

 for some reason, have 

 not partaken in the gen- 

 eral thickening of the wall, 

 been explained by supposing that a 

 slight enlargement of the original 

 wall takes place, which stretches the 

 nascent lining, so as to break or fray 

 it into slits or holes here and there. 

 But their remarkable regularity, and 

 the uniformity with which each suc- 

 cessive layer is moulded on the pre- 

 ceding, with exactly corresponding 

 interruptions (42), forbid our adopl* 



* In this way, and by taking advantage of the fact, that the secondary de- 

 posits in the cells consist in part of mineral matter, which is left behind in the 



FIG. 20. Magnified cross- section of a small portion of heart- wood of the- Plane-tree or 

 Buttonwood (Platanus occidentalis). 21. A corresponding longitudinal section, parallel with 

 the circumference, a, The dotted woody tissue ; the lower ends of the two cells to which the 

 letters are appended are divided lengthwise, so as to show the irregularly thickened calibre ; the 

 others are mostly entire, showing the dots: in the cross-section the secondary deposit is seen 

 to form indistinct layers, and some of the dots to form canals of lateral communication, b, Dot- 

 ted ducts : the middle one in the longitudinal section is obliquely jointed, c, Medullary ray. 



FIG. 22. Portion of four cells of the woody tissue, with both transverse and longitudinal 

 section, highly magnified, showing the canals or deep pits in the thickened walls, and their ap- 

 position in adjoining cells: on the cross-section the layers of deposit are more plainly visible. 



