94 



METABOLISM 



Fig. 20, Apex of a roothair in intimate union 

 with soil particles. X 240. (From the Bonn Text- 

 book.) 



have not as yet considered in detail. Roothairs when grown in soil cannot 

 maintain the symmetrical form they have in water, e. g. in Hydrocharis ; in the 

 course of their growth they encounter obstacles which they cannot avoid but 

 round which they grow. In this way the roothair becomes irregular in 

 form (Fig. 20), and, further, becomes so intimately connected with these obstacles, 

 the minute particles of the soil, by means of its mucilaginous wall that we may, 

 with every justice, speak of a growth fusion between them. In fact, the soil par- 

 ticles adhere to those regions of the roots that are covered with hairs so firmly 

 that when the plant is pulled out of the soil these regions stand out prominently 

 in contrast with the white apices which have not as yet developed roothairs, and 



also with the older parts of the root which 

 have lost them. The soil, in a word, sur- 

 rounds the root like a sleeve (Fig. 21). This 

 intimate union of soil particles and root- 

 hairs renders easy the absorption of those 

 substances which pass into solution on the 

 breaking up of the soil, and this breaking 

 up is in turn facilitated by certain excre- 

 tions formed by the roothairs. 



It has been known for long that roots in 

 nature had a corroding effect on limestone. Sachs (1865, 189) demonstrated this 

 fact by allowing plants to grow in a flower-pot in which he had placed a slab of 

 polished marble covered with a layer of clean sand. The roots of the plant were thus 

 able to grow over the marble, and, after several days or weeks, corrosion figures 

 appeared on the plate corresponding to the distribution of 

 the roots. Sachs found, as a matter of fact, that the course 

 of the chief andsecondary roots (e. g. of the bean) was mapped 

 out, wherever they were in intimate contact with the marble, 

 bya shallow rut about \ mm. broad, bordered by a hazy and 

 indistinct roughness in certain places indicative of the 

 presence there of roothairs. He obtained similar results 

 by employing dolomite, magnesite and osteolite, so that 

 magnesium carbonate and calcium phosphate are as 

 capable of suffering dissolution by roots as calcium car- 

 bonate. For a long time it was held that the roots gave 

 off free organic acids, and that these acids were the cause of 

 the formation of corrosion figures ; more recent research 

 has shown, however, that corrosion figures are due 

 primarily to the action of carbonic acid. Czapek (1896) 

 employed in his researches plates of plaster of Paris arti- 

 ficially compounded with the mineral whose solubility he 

 desired to investigate, using them in the same way as 

 Sachs did plates of marble. The two substances were 

 £^ J pounded into a paste with distilled water and then spread 



„ .,. , ... over a glass plate ; in this way a very fine flat surface 



Fijr. 21. Seedling of white i ^ • j i: • . •. "^ rr , ■ 



mustard. /, takin direct was Obtained for experiment, quite as effective as a 

 soTpartldes; //I al?er^^ein| poHshed platc of uatural rock. By this means, Czapek 

 washed. (After sachs, established that plates of carbonate and phosphate of 

 lime were corroded by roots while aluminium phosphate 

 plates were not affected, and it may be concluded that quite 

 a number of organic acids, those, in fact, in which aluminium phosphate is soluble, 

 take no part in the formation of corrosion figures ; excluding these acids we 

 have only to consider carbonic, acetic, propionic, and butyric acids. The brown 

 coloration given to congo-red demonstrates that carbonic acid at least must be 

 considered as an agent in the formation of corrosion figures, since the other acids 



Lectures on Plant Physi- 

 ology.) 



