WOOD-PAKEKCHtfMA. 77 



bringing all parts into closer relations, and strengthening the 

 whole. 



253. The cells are normally of considerable length in pro- 

 portion to the transverse diameter, and are generally more or 

 less sharply pointed (prosenchyrna proper). The most impor- 

 tant of the modified cells belonging to this system unite to form 

 long rows in which the terminal partitions are nearl}- or quite 

 obliterated, throwing the cavities into one, and thus forming a 

 cylinder, termed a duct. Between proper prosenchyma cells 

 and ducts there are numerous connecting forms which render 

 impossible any attempt at classifying them exactly. 1 



Associated with these cells, but differing in some important 

 particulars, are cribrose and latex cells, which for convenience 

 are here to receive separate treatment. 



254. Before developing the provisional classification given 

 on page 59, attention must first be directed to the peculiar 

 transitional forms constantly met with, which belong as much 

 to parenchyma as to prosenchyrna, but are more conveniently 

 examined in connection with the associated wood-elements. 



Chief among these intermediate forms must be mentioned 

 those of which Fig. 58, No. 9, may be taken as a represen- 

 tative. Here the whole structural element is isolated as an 

 elongated combination of three cells, one of which has flattened 

 ends, while the other two, attached to these ends, have their 

 free extremities pointed. In spite of their form, such cells are 

 usually described as wood-parenchyma cells. When their walls 

 are thicker, they are not easily distinguishable from septate 

 libriform cells (see 263). 



255. The forms shown in Fig. 59, No. 19, are common in 

 the wood of many plants, notably the oaks. They are rela- 

 tively small, have rather blunt extremities and thin walls. They 

 occur with these characters especially in the autumnal wood of 

 the oaks (see 395), while in the spring wood they are apt to 



1 For the satisfactory study of the relations of the elements of prosenchyma, 

 very thin sections are necessary; but for the examination of the elements them- 

 selves, recourse to some process of maceration, by which they can be isolated, 

 is always desirable. In general, there is nothing preferable to Schulze's solu- 

 tion in any strength adapted to the special case; it must be remembered that 

 the slow action of a dilute solution gives better results than the more rapid 

 action of a concentrated one. If the section to be examined is first subjected to 

 the action of the macerating solution of proper strength and then thoroughly 

 washed, it can be dissected at pleasure under a high power of a simple lens. 

 This method is always to be preferred to the ordinary one of disintegrating the 

 Whole specimen and obtaining a confused mass of separated cells. 



