PROTOPLAST OF SIEVE-TUBE ELEMENTS 95 



(1939) and tlie following resume on sieve-tube elements is in- 

 tended simply as an introductory guide. 



1. The protoplast of sieve-tube elements. One of the most 

 typical characters of the mature sieve-tube element is the absence 

 of a nucleus. Numerous developmental studies (cf. Esau, 1939, 

 p. 375 and 1941, pp. 452-454, PL 7) have shown that while the 

 yoking element possesses a normal nucleated protoplast, matura- 

 tion is accompanied by the eventual disintegration of the nucleus. 

 Prior to its breakdown, the nucleus has been observed to increase 

 significantly in size and to lovse its chromaticity. Crafts (1939a, 

 p. 176), in particular, has stressed the physiological significance 

 of the enucleate condition in the sieve-tube element as follows : 

 "The whole history of the sieve-tube portrays the intimate rela- 

 tion of the nucleus to the structure and function of the elements. 

 No student of ontogeny can fail to sense the influence that the 

 loss of nucleus has upon subsequent activity. From the begin- 

 ning of its functioning period to its death, the sieve-tube element 

 is doomed to a passive role, conditioned by its lack of nucleus 

 and consequent permeability." A peculiarity of the cytoplasm 

 of mature sieve-tube elements consists, according to the investi- 

 gations of Crafts, in its highly permeable nature. Following the 

 disappearance of the nucleus, Crafts finds that cytoplasmic 

 streaming ceases and the cytoplasm "fails to plasmolyse in hyper- 

 tonic solutions." Furthermore, the cytoplasm in the mature ele- 

 ment loses its former ability to accumulate neutral red, a vital 

 stain. Crafts (1939a, p. 175) interprets these facts as indicating 

 the highly permeable character of adult sieve-tube elements. As 

 Esau (1939, p. 403) has pointed out, the enucleate and permeable 

 cytoplasm of mature sieve-tube elements is not to be regarded 

 as "dead." This appears to be demonstrated by the continued 

 deposition of callus on the sieve-plates during late stages in 

 ontogeny. The contents of mature sieve-tube elements of certain 

 species (e.g., Cucurhita) consist of slimy proteinaceous material 

 which, in sections of phloem treated with heat or alcohol, may 

 coagulate on the sieve-plates and in the lumen to form funnel- 

 shaped structures known as slime-plugs. (Cf. Eames and Mac- 

 Daniels, pp. 193-194, Figs. 90C and 91C ; and Esau, 1939, pp. 

 379-383). The most recent evidence supports the belief that 



