34 



GENERAL DIFFERENTIATION OF THE PLANT-BODY 



its apex, the apical cell, and the branches, which are constructed like 

 the chief axis, arise in progressive serial succession from the cells thus 

 produced. At a later period division begins also in these segment-cells, 

 and from the new cells thus developed branches are formed which are 

 intercalated between those previously existing. If we suppose now that 

 this formation of intercalary branches is suppressed, as is the case in 



other species of Cladophora, for example in 

 C. prolifera (compare Fig. 9), we should 

 retain the scheme of formation of lateral 

 organs which is the most widely spread amongst 

 plants, namely, that of the progressive origin 

 of new formations. 



A difference between the construction of 

 chief axis and of lateral shoots is often concur- 

 rent with the branching, as has been already 

 pointed out in the case of Hydrurus. The 

 chief axis has, as in that species, to discharge 

 a mechanical function different from that 

 belonging to the lateral axes and it also 

 stands in a definite metabolic relationship 

 to them it supplies material to the lateral 

 axes and at the same time draws upon them. 

 When the lateral axes are pressed closely 

 together and envelope a chief axis they make 

 the work of assimilation more difficult for 

 it because they intercept the light. It is 

 not surprising therefore that we meet with 

 differences between chief and lateral axes. 

 We find this, for example, amongst Algae 

 which are copiously branched, like Drapar- 

 naldia and Chaetophora ; at a certain age 

 of the plant when the cells of the lateral 

 axes continue an undisturbed growth, those 

 of the chief axis are incapable of dividing, 

 of producing swarm-spores, and of assimi- 

 lating in a comprehensive manner, and similar phenomena may naturally be 

 repeated between lateral axes. As the plant-body increases in size there 

 is, as Herbert Spencer 1 has so well said, a tendency to the development 

 of an inequality between its members, and this shows itself especially in 

 a difference between chief axes and lateral axes. 



Besides this we not infrequently observe in the Algae that the cell 

 which gives rise to a branch lags behind the others in its growth either 



1 Herbert Spencer, Principles of Biology, i. p. 214. 



FIG. g. Cladophora glomerata. Por- 

 tion of thallus. The branching is in 

 regular progressive serial succession 

 towards the apex. Magn. 48. Lehrb. 



