BOTANY AND MORPHOGENESIS 1 23 



of differentiation that occur during the life cycle. In many cases it has been 

 found that cuttings from different parts of the plant, and thus from different 

 developmental levels, produce plants with persistent differences. Cuttings 

 from the flowering branches of English ivy, for example, produce plants very 

 different from ones made from the earlier vine-like portions. Cuttings from 

 seedlings will often produce plants unlike those taken from adult parts. 



The paradox of differentiation is that it occurs in living stuff that is 

 genetically uniform. One great contribution that plant science has made to 

 morphogenetic problems is the proof that this is so. Cuttings can be pro- 

 duced not only from stems but from roots, leaves, hypocotyls, inflorescences, 

 and even floral parts. In many cases, individual cells or groups of cells from 

 parts of the plant body can be induced to divide, become meristematic, and 

 ultimately regenerate new plants. This can be done by applying growth sub- 

 stances, isolating pieces of plant tissue, or in other ways. The plants produced 

 from these various regions are all like the parent plants (save that the cells 

 are sometimes polyploid), thus indicating that all parts of the plant are 

 genetically alike. How the differences arise from these identical cells is obvi- 

 ously a major problem. 



The very beginning of differentiation can often be traced, in plants, to a 

 particular cell division where the two daughter cells are dissimilar, as in the 

 origin of root-hairs, stomata, glandular cells, and other structures. Here the 

 process of differentiation is pin-pointed, so to speak, and offers an excellent 

 opportunity to find just how it begins and what causes it. The relative rigidity 

 and stable position of plant cells make them particularly favorable for such 

 studies. 



In more complex cases the origin of a new tissue within a relatively un- 

 differentiated cell mass may be observed, as in the development of the endo- 

 dermis, the veins within the leaf mesophyll, or the fibrous net in a cucurbit 

 fruit. All such examples offer opportunity to study differentiation more di- 

 rectly and easily both visually and by microchemical methods than can 

 usually be done in animal tissues. 



Plants have notable powers of regeneration, and the student of morpho- 

 genesis has long used this ability in his investigation. Several examples of 

 this have already been cited, notably the ability to grow whole plants from 

 single cells or small groups of cells. 



The regenerative process in animals usually consists in remolding an organ, 

 experimentally deprived of some of its normal structures, into a complete 

 whole. In plants this process is more various. Sometimes it resembles that 

 in animals, as when the tip of a root restores itself when only a small piece 

 is cut off. Sometimes a larger portion of an organ may be restored, as when 

 a leaflet is replaced after its removal. Far more commonly, however, there 

 is no true repair of a structure but instead a restitution of it by the develop- 

 ment of new young parts, often from a callus layer. This is what normally 



