164 ELEMENTS OF STRUCTURAL BOTANY. 



left in the cross-partitions between them, sieve-cells are 

 formed. Here, again, the thin spots finally disappear, 

 thus practically uniting adjacent cells. 



271. It sometimes happens that the thickening takes 

 place throughout the length of a cell but in its angles 

 only. Cells of this kind, which are often found im- 

 mediately under the surface of the stem in the higher 

 plants, are called collenchyma cells. 



272. Besides the markings on the inside, cells often 

 show markings on the outside. The pollen-grains of the 

 Mallow, for instance, are seen under the microscope to be 

 covered with pointed projections. Other pollen-grains, 

 also, exhibit outside markings of different sorts. 



273. The thickening deposit may be so excessive in 



some cases as to almost completely fill up 

 the cavity of the cell (Fig. 225). The 

 shells of nuts and the tough coatings of 

 seeds consist of cells of this kind ; but even 

 in these cases the wall may be seen to be 

 traversed by slender pores or canals, either 



simple or branched, radiating from the centre of the cell. 



To these hardened cells the name sclerenchyma is applied. 



; 274. The Contents of Cells. If you look at Fig. 



215, or, better still, if you have the opportunity of 

 viewing a Moss-leaf through a good microscope, you will 

 see that in the protoplasmic lining of the cells there 

 are numerous greenish, rounded granules. These are 

 the bodies to which the green parts of plants owe 

 their colour. They are called chlorophyll-granules, and 

 consist of protoplasmic matter in which particles of green 



Fig. 225. Sclerenchyma, the cell-cavity being almost obliterated. (Gray.) 



