322 



STUDIES IN SYMBIOSIS, 11., 



many cases numerous bacteria. There is no definite algal zone as in Cycas 

 revoluta roots, although sometimes small unicellular algae occur amongst the 

 superficial cells of the root. Surrounding the cortex there is a very distinct 

 sheath of large, thin-walled, practically papillate, and radially elongated cells, 

 with inter-cellular spaces. The outermost cells of this sheath, especially along 

 that part of the root surface which has developed through soil particles, are 



Text-fig. 4. — Transverse section of tubercle showing diarch stele, endo- 

 dermis, cortex (c), and sheath (s) . (x 50). 



Text-fig- 5. — A portion of the sheath (s), many of the cells of which 

 have a nucleus, starch grains (s.g.) and bacteria (b)- A 

 small group of unicellular algae (A) is seen in the outer 

 sheath-cells. The small cells at the base of the sheath 

 are meristematic (m). (x 265). 



frequently crushed, more or less disorganised and devoid of protoplasmic con- 

 tents. The inner cells of the sheath contain a peripheral film of cytoplasm, a 

 nucleus and generally several small starch grains. Bacteria frequently occur on 

 the surface of the sheath and inside certain of the sheath-cells (Text-fig. 5, b). 

 There is no typical cork formed on the tubercle of Macrozamia. 



In a longitudinal section (Text-fig. 6) it will be seen that the sheath ex- 

 tends completely round the tip of the root, and forms a definite and pei-sistent 

 root-cap, somewhat different in structure from the cap of a noi-mal root. The 

 papillate sheath-cells at the actual tip of the root are generally very regular in 

 form and arrangement, and have not suffered from pressure. Beneath this 

 sheath, at the apex, there is developed a very massive meristem, tlie cells of which 

 are small, and contain very granular cytoplasm, a large nucleus, and several 

 starch gTains. The calyptrogen and dermatogen are not defined as in the normal 

 root. The periblem and plerome merge into the primordial mass at the apex. 

 The meiistematic cells are absolutely free of bacteria, although cortical cells 

 and sheath-cells near by contain these 



