EXPERIMENT STATION BULLETINS. 427 



Longitudinal sections are apparently indispensable and have revealed struc- 

 tures not demonstrable by any other method. 



Some erroneous views held by botanists have been dispelled by observa- 

 tions of many thousands of sections of bean, tomato, potato, and other 

 plants. Some botanists have taught that nuclei are not present in the sieve 

 tubes. Nevertheless, they occur regularly in sieve tubes in all stages of 

 development and are easily demonstrated by a study of longitudinal sections. 

 Plastids, which are also supposed to be absent from the sieve tubes, are 

 often found in great numbers. This is especially true of cucurbitaceous 

 material. In mosaic plants the plastids may almost surround the nucleus in 

 cells of the sieve parenchyma and adjoining tissues. Abnormal nuclei are 

 also present throughout the phloem of diseased plants and may easily be 

 mistaken for possible organisms by any one not familiar with the material. 

 Chondriosomes are numerous in the parenchyma cells of some of the plants 

 studied and due to their great resemblance to large bacteria, especially 

 in stages of division, are susceptible to erroneous interpretation. 



ORGANISMS IN DISEASED PLANTS 



Bean Mosaic 



The sieve tubes of bean plants are remarkably free from food particles 

 that take the stains used in this work. This is not true with all plants since 

 sections of mosaic material from plants of other kinds have shown these tubes 

 to be filled with deeply staining granules of appreciable size. Because of the 

 clarity of the phloem tissues when properly destained the bean offers excellent 

 material for a study of this kind. 



Longitudinal sections of stems or petioles from mosaic plants, killed and 

 fixed in protozoan killing and fixing solutions^ and stained with Haidenhain's 

 iron-alum haematoxylin, show the constant occurrence of biflagellate organ- 

 isms in the sieve and sieve parenchyma cells. They are fixed and stained 

 powerfully by these solutions and by proper destaining all color can be re- 

 moved from all plant structures except the nucleoli and chromatin and still 

 remain brilliant in the organisms. The organisms are relatively 'large, some 

 individuals being 18 microns in length, and are easily seen with ordinary' 

 high power dry lenses. Including the fully extended flagella some indi- 

 viduals will measure 50 microns. These flagellates are scattered throughout 

 the sieve and adjoining parenchyma cells in the stems and petioles of diseased 

 plants. They could not be found in the tissues of healthy plants, of which 

 many were studied, using the same methods of killing, fixing, dehydrating, 

 sectioning and staining. Fig. 2 is a photomicrograph of a longitudinal sec- 

 tion through petiole phloem of a diseased plant and illustrates the distribution 

 of the organisms. 



The following are the principal forms that have been observed in sections 

 of diseased tissues: (1) Large, elongated, biflagellate, tapering, or cigar- 

 shaped individuals with the form of the body resembling organisms belonging 

 to the genus Leptomonas, the flagella being attached at opposite poles. This 

 is the most common type. (2) Ovaliform biflagellates of the same type. 

 (3) Deep-staining, shghtly elongated bodies, surrounded by a lighter en- 



^Various standard solutions containing Hg CI2, were used. 



