326 



THE CELL 



The tender root-hairs of plants exhibit a characteristic struc- 

 ture at their growing points. Hence the nucleus, as long as 

 growth continues (Fig. 162 A), is situated at the free end, whilst 

 when the hairs are old and fully developed, it is higher up. 

 When a root-hair is developing out of an epidermal cell, a protu- 

 berance is always formed upon that part of the external wall, 

 which is situated over the cell-nucleus (Fig. 162 B). In many 

 plants (Brassica oleracea) the root-hair cell may form branches, 

 into one of which the single nucleus enters. This one becomes at 

 once the richest in protoplasm and also the longest, whilst the 

 other branches leave off growing. 



The hairs that grow above ground, differ from the root-hairs, 

 in that they exhibit a basipetal, or intercalary growth, as Haber- 

 landt has established by measurements. In consequence of this, 

 the nucleus is not situated at the apex, but near to the place, 

 where the secondary, basal growing-point is situated, and where 

 longitudinal growth persists longest. 



Stellate hairs (Fig. 163) are peculiar, unicellular structures, 

 which split up at their peripheral end into several radially diver- 

 gent branches. Under these circumstances the nucleus, as long as 

 the formative processes continue, is situated in the middle of the 

 radiation, but after growth is finished it returns to its former 

 position near to the base. 



Confirmatory evidence of this participation of the nucleus in the 

 formative processes is furnished us by the examination of Fungi 



and Alg-a?. In the multi-nucleated 

 hyphae of Saprolegnia lateral 

 branches develop ; these are always 

 found immediately over a nucleus, 

 which is situated close to the cell- 

 wall. In Vaucheria and other multi- 

 nucleated Alga?, as in the higher 

 plants, special growing points are 

 present, at which growth chiefly oc- 

 curs ; at each of these, immediately 

 underneath the cellulose membrane, 

 there is an accumulation of small 

 nuclei, after which comes a layer 

 of chromatophores ; in the remain- 



Fig. 163,-Young stellate hair of An- port i ons of the Cell the positions 



trietia delloidea (afterHaberlandtjPl.il., . 



Fig. 28). of these bodies are reversed. 



