12 Mitosis in Pellia 



THE CENTROSOME PROBLEM 



The centrosome^ problem is one of extreme difficulty, and perhaps the difficulty is 

 greater for the botanist than for the zoologist. At least, the difficulties are different in 

 the two cases. That there are in animals well-defined centrosomes which function as 

 organs of nuclear division, all investigators agree, and animals or tissues in which cen- 

 trosomes do not occur are regarded as exceptions. The existence of the organ is not a 

 serious problem; rather, the more recent investigations have sought to establish the 

 permanent or transitory character of an organ which all admit to be present during 

 mitosis. In plants, on the other hand, even the existence of a centrosome is a problem 

 which must be considered separately for the different groups. 



It is of interest to note that centrosomes in plants were first observed in diatoms 

 in 1886 by H. L. Smith (31). When Guignard in 1891 published his classic paper 

 on fertilization, botanists at once accepted the results and confirmatory accounts 

 appeared. Strasburger (33) found centrosomes in Larix, Humphrey (18) in Psilotum, 

 Mottier (22) in Delphinium, Schaffner (26) in All sma and Sagittaria, Campbell (3) in 

 Equisetum, Lauterborn (21) and Karsten (20) in diatoms, and other investigators 

 reported centrosomes in various forms ranging from the algae up to the flowering 

 plants. In fact, the centrosome seemed to be as universally present in plants as in ani- 

 mals. Belajeff (1) and Farmer (11), however, failed to find centrosomes in Lilium. At 

 the same time Strasburger (35), directing a remarkable group of investigators, attacked 

 the problem in all the principal groups of plants. Those who studied Thallophytes 

 found centrosomes, but those who studied Pteridophytes and Spermatophytes not only 

 found no centrosomes, but, in tracing the origin of the multipolar spindle, they found 

 conditions which seemed to preclude any such bodies. Just as the discovery of cen- 

 trosomes was followed by confirmatory accounts, the multipolar spindle and the non- 

 existence of centrosomes in the vascular plants received immediate confirmation. 

 Guignard, Schaffner, and others still continued to find centrosomes in flowering plants, 

 although these bodies, as represented in the figures, became noticeably less conspicu- 

 ous than in earlier accounts. In Guignard's (15) recent studies of fertilization no 

 centrosomes are represented in the figures, and no reference to any such structures is 

 made in the text, even during the stage at which the famous " quadrille of the centers" 

 was formerly (14) described. The fact that the great majority of cytologists, with the 

 most approved technique and provided with apochromatic immersion lenses fail to find 

 centrosomes in flowering plants, added to the fact that the mode of spindle-forma- 

 tion both in reproductive and in vegetative cells does not require the participa- 

 tion of a centrosome, makes the evidence overwhelming that the centrosome, as an 

 organ of division, does not exist in this group. 



In regard to the Pteridophytes, the evidence is similar, but not nearly so extensive. 

 The blepharoplasts of Pteridophytes and Gymnosperms will be considered later. 



2 In referring to flowering plants no attempt has been spheres. In describing mitosis in liverworts some writers 

 made to distinguish between centrosomes and centro- have used these terms indiscriminately. 



338 



