GROWING POINT 45 



of the dermatogen, or in some cases, of the dermatogen 

 itself, or, in still other cases, by the division of some of 

 the cells of a common mass of initial cells from which the 

 root cap as well as epidermis, periblem and plerome 

 arise. On the growing points of stems, the new branches 

 arise by the formation of secondary growing points at 

 the side of the main one, these having the same 

 general plan. Those that produce the leaves often grow 

 faster than the mai growing point and sur- 

 round and protect it, thus forming a bud. 



60. As the growing point progresses, the 

 cells formed in it come to lie further and 

 further from the apex. They increase in size 

 and, after a while, cease to divide. Certain 

 of the cells remain meristematic a long while; 

 others become elongated, i.e. cease early to di- 

 vide transversely, and eventually become 

 transformed into fibrous, tracheary, sieve tis- 

 sue or collenchyma. Some cells merely enlarge and 

 become parenchyma. Thus, near the tip the cells will 

 be found to be all meristematic, but further back, various 

 kinds of tissues may be found. 



Laboratory Studies, (a) Make a longitudinal section of the 

 end of a branch of the marine alga, Codium tomentosum. Here 

 the growing region is not a few cells as in a true growing point, 

 but each filament elongates at the apex without the production 

 of cross walls. Many of the Red Seaweeds (Rhodophyceae) 

 show the same type of apical growth except that transverse 

 walls are formed near the apex of each filament (e.g. Melobesia, 

 Nemalion, etc.). 



(6) Examine the end of a shoot of Sphacelaria, one of the 

 Brown Seaweeds. Here there is a single apical cell which divides 

 by a transverse partition, the segments thus formed dividing 

 longitudinally and transversely. 



(c) Make a thin longitudinal section through the growing 

 point of a moss or of a stem or root of a fern or horsetail 



