CELL-DOMINIONS IN THALLOPHYTA 



35 



temporarily or permanently. If this happens regularly the chief axis 

 itself will exhibit a differentiation ; the places upon it whence lateral organs 

 spring, the nodes, behave differently from the internodes, and the difference 

 is in many cases extremely sharp, for example, in Chara (Fig. 10). The 

 cause of this may perhaps be that the formation of a lateral organ 

 originally brought about a direct arrest of the zone of the chief axis 

 from which it sprang. 



We have thus arrived at a plant with a vegetative 

 point and a regular evolution of lateral organs ; we 

 have further seen how, frequently through the branch- 

 ing, a more or less marked difference between chief 

 axis and lateral axes may be brought about ; and 

 we have now to examine the different constructions 

 of the lateral axes themselves. 



It is not at all unusual to find lateral axes dis- 

 tinguishable into long shoots and short shoots. The 

 external differences between the two kinds of shoots 

 are a consequence of the fact that the long shoots 

 have unlimited growth whilst the short shoots have 

 limited growth and therefore appear like arrested 

 conditions of the long shoots. A difference of function 

 however is bound up with this external difference 

 in form ; . the long shoots are the instruments of the 

 special branching of the plant ; the short shoots are 

 chiefly organs of assimilation. Thus in Chara the 

 ' leaves ' which are arranged in an apparent whorl are 

 merely short shoots which differ also from the long 

 shoots in having a simpler structure. We shall meet 

 with these differences again among the higher plants ; 

 it will suffice to recall here as examples the well- 



, .... - ,, . r . , FIG. 10. Chara fragilis. 



known relationships of configuration ot species of Portion of shoot. Thewhoried 



T1 . - _, ' leaves ' which bear the sexual 



Pinus and ot many Lacteae. organs are short shoots. 



.... . . . . , , Natural size. Lehrb. 



Alike in the higher and lower plants the short 



shoots and the long shoots primarily differ only quantitatively not 

 qualitatively. A lateral axis which may develop into a long shoot under 

 favourable conditions will, under unfavourable conditions, become a short 

 shoot. In those cases in which long shoots and short shoots are more 

 sharply separated their place of origin is also different, and there is 

 besides an increase in the difference in structure which they exhibit. 

 A few illustrations of this may be given. 



The Sphacelarieae x , which belong to the Phaeophyceae, furnish us 



1 See Geyler, Zur Kenntniss der Sphacelarieen, in Pringsh. Jahrb. iv. p. 479 ; Pringsheim, Uber 

 den Gang der morphologischen Differenzierung in der Sphacelarieenreihe, Gesammelte Abhand- 



D 2, 



