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In making a Ionc[itudinal section of a terminal bud with a 

 sharp knife, the operator quickly learns to detect the starch 

 pith as soon as it is reached in the downward passage of the 

 instrument. The bud proper is cut with eas?, but there is a 

 sensation known to grafters and others as that of "grittiness/* 

 which is due to the resistance to the knife caused by the many 

 thick walls of the starch-bearing cells. This is one of the 

 most important modifications of the ordinary cell wall, and 

 consists of a thickening due to the intercalation into it of a 

 substance called lignin, which chemically differs somewhat 

 from cellulose, and adds materially to the hardness and dura- 

 bility of the tissue lignificd. This power of resisting external 

 influences is gained, however, at the loss of much elasticity. 

 Lignified cells do not abound in protoplasm, but water passes 

 freely through their walls. All permanent, inactive tissue 

 may become lignified, and when this process is freely carried 

 out it yields the durable heart-wood so familiar in many kinds 

 of timber. The subject is of interest because it helps to ex- 

 plain the matter of ^'grit," or *' grittiness,"so frequently spoken 

 of by those who cut the twigs of various sorts of fruit-trees. 

 The inflexibility of tips of certain plants is due in most parts 

 to the large amount of lignin their pith contains. 



The free end of a mature twig consists of a cone of mi- 

 nute, thin-walled cells, upon the outside of wdiich the small, 

 imperfect leaves, as bud-scales, arise in regular order, and, 

 overlapping each other, enclose the tender growing point of 

 the twig. From the scales, bundles of fibers and vessels de- 

 scend and form a thin ring of wood around the pith just below 

 the growing point. At this portion of the twig the pith 

 makes up the greater part of its substance. In short, the bud, 

 the stem, and for a quarter to a half inch below it, are com- 

 posed of soft tissue, easily crushed with the thumb and finger. 

 But below this the large central pith is particularly rigid, due 

 to the unusual thickening that has taken place in the walls of 

 the cells. If the reader will bear in mind that the wood zone 

 is very thin at the upper part of the twig, and that the bast is 

 almost wanting, it will be evident that the pith alone must 

 give the rigidity found near the extremity of matured twigs. 

 From the shape of the nearly spherical cells it follows that 



Vol. II. — 2. 



