GROWTH 143 



ward on the crest of the tissue which it creates. The growing- 

 point may perhaps be compared with the coral animals, which 

 form only a very thin layer at the surface of the coral reef but 

 by their activity build the reef farther and farther outward and 

 are carried out upon it; or it may be compared to a brick-layer 

 constantly adding bricks to the top of a wall and being carried 

 himself high in air by the wall which he has made. 



This method of growth at a definite point or laj^er, through 

 the activity of a meristem, which is so characteristic of plant 

 tissues and so different from that employed in the growth of 

 animals, has certain consequences worthy of mention. It writes 

 in the body an almost complete history of the plant's growth and 

 development, for many of the first-formed tissues are still present 

 (unless lost through decay), buried in the successively later 

 accretions which have been added from time to time. A careful 

 internal and external examination of a tree trunk, for example, 

 enables us to tell almost exactly how tall and how thick the tree 

 was at any year in its past histor}^ An understanding of the 

 location and activity of growing-points is desirable in the practice 

 of the various methods of grafting and budding, for these are 

 Qecessarily concerned with a manipulation of the meristematic 

 regions. 



There are two general types of growing points in most plants — 

 terminal and lateral. The former, which develop at the tips of 

 roots and stems, cause an increase in the length of these organs, 

 and through their activity the stem grows tall and its roots spread 

 farther into the soil. The latter, of which the cambium is the 

 characteristic example, forms a ring or sheath of growing tissue, 

 encircling the root and stem throughout their entire extent and 

 causing these organs to increase in thickness. 



Terminal Growing-points. — The growing tip of a root furnishes 

 a good example of a terminal growing-point, and a brief study of 

 this region will perhaps enable us to understand more clearly just 

 how such a type of meristem functions (Fig. 71). At the very tip 

 of the root is the root cap, a body of dead cells continually renewed 

 from the growing-point within as they are sloughed off by friction, 

 and which protects the delicate root tip as it is forced through the 

 soil. Just back of this is the meristem itself, a relatively small 

 mass of tissue usually not more than two or three millimeters in 

 length, and composed of small, thin-walled and richly protoplas- 

 mic cells. Cell division takes place in this region and here alone. 



