Problems of Plant Physiology 39 



readily seen that scarcely any part of the soil will escape penetration 

 by one or more rootlets. Many relationships, symbiotic and otherwise, 

 in connection with the root system await solution as are also various 

 problems connected with the methods and effects of aeration. 



The direction and rate of movement of water through the cortical 

 cells of the root to the fibro vascular bundles still requires elucidation 

 (7, p. 84). Roots generally are thicker the higher they arise from a 

 stem (8, p. 126) but become smaller and smaller as they ramify through 

 the soil. Here their function is chiefly one of absorption as can be 

 shown by aniline dyes (1, Bd. I p. 134). 



The ability of certain plant parts to again put out new roots is 

 well known. "A leaf, however, of Phaseolus vmltiflorus cut off at the 

 pulvinus" and put in water may form roots and live for months and this 

 is also true of Ficus elastica (8, p. 164). This power is possessed by 

 the cotyledons of various plants even if cut into pieces (8, p. 164). So 

 we have here a line of study to ascertain the cause of these well known 

 differences in plants. The germination of the seeds of citrus plants and 

 the Cucurbitaceae within the closed fruit is not rare and may perhaps 

 be due to temperature conditions. Even when the seeds turn green 

 this may be due to pathogenic conditions, or to a disturbed state of 

 nutrition (1, Bd. I. p. 318). That some plants form active chlorophyll 

 in darkness is well known. Cotyledons make but little or no food while 

 filled with reserve materials even if green (9, p. 596). Although some- 

 times colored when exposed to light, roots generally are colorless. Nev- 

 ertheless in Menyanthes a small amount of chlorophyll is produced 

 (8, p. 166). 



The importance of root-hairs merits further study. Schwartz (10) 

 has shown that there may be as many as 230 root hairs per square 

 millimeter and that these increase the absorbing surface as much as 

 12 times. The location and development of the root hairs give an ex- 

 cellent indication of the capabilities of the root for absorption. Various 

 external conditions influence their development. According to Schwartz 

 they are absent in Allium cepa, Cicer arictinum, Cucurblto i^epo, Helian- 

 thus annmis, Phaseolus vmltifloms, P. communis, Ricimis covimiinis 

 and Zea mays when the plants are grown in water. Some water plants 

 do not produce root hairs except when they send their roots into soil 

 (11, p. 194). Some root hairs may branch and remain unicellular 

 (11, p. 25) as those of Brassica napiis. There is the question whether 

 new root hairs are ever produced between pre-existing ones or whether 

 they always arise acropetally (11, p. 195). Snow (12) and others (13) 

 have made valuable contributions to this topic. 



Great interest attaches to the pegged and smooth rhizoids of Mar- 

 chantia. If, according to one view, the pegged ones increase the ab- 

 sorbing surface and are of no mechanical assistance, we must also 

 consider seriously the theory of Kamerling as to the service of the gas 

 bvibbles. At any rate this matter deserves thorough investigation (11, 

 p. 202). The work on an individual root hair is small and it has been 

 estimated that each hair will only absorb about .000,001 milligram 

 (14, p. 242). However, their united action is great. Many problems 



