44 



VEGETABLE PHYSIOLOGY 



of a young plant is consequently much smaller than that 

 of an old one. The difference between the condition 



of a stem at two periods may be 

 seen by comparing fig. 40, A, B, and 

 c ; A and B show diagrammatically 

 the arrangement of the supporting 

 and conducting tissue at an early 

 stage of its life, while c indicates 

 the condition several months later. 

 During the interval a large forma- 

 tion of secondary vascular tissue 

 has taken place, and new bundles 

 have been intercalated between the 

 original ones. 



The structure of a coenocyte 

 shows a similar mode of forma- 

 tion of the skeleton to that of 

 a multicellular plant-community. 

 In this case, however, the several 

 protoplasts are not furnished with 

 separating walls. The only skele- 

 ton is the external membrane which 

 limits the whole structure, and 

 which is formed by the conjoint 

 activity of them all. In compound 

 or septated coenocytes we have in 

 addition certain transverse walls 

 crossing the interior and giving a 

 greater degree of strength to the 

 whole body. These separating walls 

 have a similar origin. 



The primary cell-wall which 



clothes the unicellular plant, and which serves as the original 

 supporting membrane of the separate protoplasts of a com- 

 munity or colony, is, when first formed, a clear, trans- 

 parent, extensible, and elastic membrane. It remains in 

 contact with the protoplasm so long as the latter is living. 



FIG. 41. EMBRYO OP Orobus 



AT THE END OF A LONG 

 SUSPENSOE, THE TWO SEG- 

 MENTS OP WHICH HAVE A 



CffiNocYTic STRUCTURE. 

 (After Guignard.) 



The rounded bodies in the seg- 

 ments of the coonocytes are 

 the nuclei of the protoplasts. 



