Meristems 59 



In a few of the lower vascular plants, notably Lycopodium, no single 

 apical cell can be distinguished, and the same is true of most gymno- 

 sperms and angiosperms. Instead, the meristem at the tip of both root 

 and shoot consists of a considerable group of embryonic cells. Many of 

 these divide actively during the growth of the plant, and they produce 

 all the tissues of the axis (save those formed by later cambial growth) 

 as well as the leaves and branches. 



Much attention has been paid in recent years to the structure, organiza- 

 tion, and activity of apical meristems, particularly in the ferns and seed 

 plants. These regions of persistent embryonic character have often been 

 compared to animal embryos. Botanists have tried to find a correspond- 

 ence between their structure and that of the parts that grow from them 

 so that the developing plant might be analyzed in terms of embryonic 

 regions, as zoologists have been able to do by using the germ layers 

 established in the animal embryo. A wide variety of plant meristems 

 have been studied and compared, but differentiation into layers as pre- 

 cise in their fate as ectoderm, mesoderm, and entoderm seems rarely to 

 occur. Some botanists, however, do regard meristematic layers as true 

 germ layers (Satina, Blakeslee, and Avery, 1940). 



Although these apical meristems do not provide a precise classification 

 of plant tissues, much information of importance for morphogenesis may 

 be derived from them. Observation of the way in which meristems have 

 produced the various tissues and organs of plants has been of service in 

 the solution of problems in growth, differentiation, and phyllotaxy. Plant 

 meristems offer the great advantage that a single plant may produce 

 many of these embryonic regions, which are thus genetically identical. 

 Though small, meristems are open to direct experimental investigation, 

 and this has already provided results of much morphogenetic significance. 



The apical meristems of shoot and root, though alike in many respects, 

 show certain characteristic differences, and further consideration of their 

 structure and activity will be more profitable if each is considered by 

 itself. 



THE SHOOT APEX 



The length of the growing region in the shoot is considerably greater 

 than in the root and may often extend over a region of several centi- 

 meters. Cell division persists longer in some tissues than in others and 

 usually stops first in the pith. No very sharp line is to be found between 

 the developing region and the mature portion behind it. The strictly 

 meristematic zone, however, where cell division chiefly occurs, is usually 

 limited to a few millimeters or less, and most growth of the stem in length 

 results from cell elongation back of this. 



