298 The Phenomena of Morphogenesis 



Gautheret (1945) showed that with ordinary plant material, as in carrot, 

 endive, and various woody plants, a callus must first be allowed to de- 

 velop from an excised piece and that this callus could then be cultured. 



There are many differences between species and between different 

 parts of the same plant as to growth and structure of the culture pro- 

 duced. Pieces of tissue from the cambial region, for example, which 

 retain some degree of organization for a time, gradually lose it in 

 successive transfers. Some cultures finally become essentially homo- 

 geneous masses of parenchyma. In a few cases, however, patches and 

 whorls of tracheid-like tissue appear in these, as do meristems of roots 

 and shoots. The conditions under which this differentiation occurs vary, 

 depending in part on the source of the material and especially on the 

 character of the nutrient medium and the age of the culture. 



It is now possible to produce plant cultures where the cells are not 

 united into masses but grow and divide as individual cells, much as they 

 do in certain animal cultures. This may be thought of as representing 

 the lowest level of plant organization, the loss of all relationships above 

 the single cell. Among those who have done pioneer work in this 

 field are Muir, Hildebrandt, and Riker (1954, 1958); de Ropp (1955); 

 Nickell (1956); Reinert (1956); Torrey (1957a); and Tulecke (1957). 



Of particular interest for morphogenesis is the culture work of Steward 

 and his collaborators (1958 and p. 75). Using a basic liquid culture 

 medium supplemented by coconut milk and continually rotated, they 

 grew small explants of carrot tissue taken from the phloem of the 

 root. Single cells here often became separated and floated freely, dividing 

 very irregularly and growing into small cell aggregates. When these 

 reached a certain size, the cells at the middle of the aggregate began to 

 show differences from the rest, and some developed as tracheid-like struc- 

 tures, surrounded by a ring or sheath of cambium-like cells. From this 

 center, one or more root meristems developed. When such an aggregate 

 is grown on the surface of an agar medium it produces a shoot meristem 

 and in time a carrot plant. So far, a whole plant has not been produced 

 directly from a single cell but only by way of a cell aggregate. It is sig- 

 nificant that although bits of carrot phloem tissue, placed directly in 

 culture with coconut milk, may form large masses of callus-like tissue, 

 they much less readily produce roots and shoots, perhaps because of 

 inhibiting substances still present in them. 



Radical though the changes are which plant tissues display when 

 cultured, there is no evidence that any permanent or genetic effect is 

 produced in them or any new developmental potencies induced. A con- 

 siderable degree of morphogenetic control has been relaxed, just as it 

 has been in amorphous galls. 



It is evident that there is an immense amount of information available 



