34 



COMPENDIUM OF GENERAL BOTANY. 



The following considerations will be of interest more especially 

 from the physiological point of view : 



I. This form of cell-wall thickening is usually met with in iso- 

 lated cells whether they are bounded by air or water. In the 

 atmosphere we find the spores of cryptogams, pollen-grains of 

 phanerogams; in water the unicellular algae. The thickenings 

 evidently serve definite purposes, some of which are as yet not made 

 clear. The spines of pollen-grains assist in adhering to the stigma 

 where the formation of the pollen-tube begins ; spines of spores no 

 doubt serve the purpose of fastening them to the soil or other sub- 

 stratum. They also favor the transportation of pollen-grains by 

 insects. 



II. Linear thickenings are of the greatest importance for 

 certain life-functions of the more highly organized plants. The 

 remaining portion of the cell-wall of such elements is usually very 

 thin. The thickenings serve as mechanical support against radial 

 pressure. To understand their use it is necessary to be somewhat 

 more explicit. The figures (Fig. 15) give a fair idea of the external 

 appearance of these elements and require but little explanation. They 

 represent portions of vessels. By vessel is meant a row of cells 



FIG. 15. Schematic lateral view of various vascular forms. 



In a and 6 the proximal surface is supposed to be removed, showing the distal halves of the 

 rings with their cut ends. 



converted into a tube, having characteristic cell- wall thickenings 

 and being essentially a lifeless structure. They are formed by 

 the partial or total absorption of the transverse partitions of cell- 



