-U / A' U TE S TRL'C TURK. 



tfie product ion of jiollen, liowovcr. ami of the spores of 

 vascular ciypto.uiiins. four iirw iniclfi ure formed in the 

 cell, and the protoplasm collects about these, eventuully 

 secreting walls, so that four new and complete cells nro 

 formed within the orifiinal one. and those sooner or lati-r 

 make their escape. This mode is known us //re <v//- 

 I'ormation. Fij;. 272 shows the formation of pollen- 

 j^rains of Hollyhotk in four stages. In tlie production of 

 the endospoiin colls in the emhryo-sac, and the spores of 

 many of the lower plants, a similar process goes on ; hut 

 here the division of the nucleus is not limited to four i)or- 

 tions, as in the cases just mentioned, but may be carried 

 on to an indefinite extent. 

 In some lower plants the entire contents of two adjacent cells 

 may coalesce to form a siniifle new cell. This mode is 

 known as conjuijation. Fi-. 2T.> shows tliis process in 

 Spii-ogyra. At a the fusion of the protoplasm is goini.' 

 on ; at 6 it is complete. Also, tlie contents of a cell nniy 

 contract and develope a new cell-wall, a process known 

 as the rejuvenescence, or renewal of a cell. 



Tissues. 



An aggregation of similar cells is called a fissnr. Originally, 

 every part of a plant consists of vierishvi, that is, of cells 

 capable of dividing. But changes set in, as we have seen, 

 at a very early stage, and eventually' all the cells assume 

 jiernianent forms, some developing in one way, others in 

 quite a different way, according to the function of each 

 particular part. So that in any given plant we find tis- 

 sues, or groups of cells, of very various kinds, and verj' 

 different arrangements of these tissues in different ca.si-s. 

 By examining sections taken in succe.ssion from the grow- 

 ing point backwards, every degree of change from meri- 

 stem to permanent tissue may be made out. 



In the growing parts of all plants, in the pulp of fruits, in tho 

 pith, in the green parts of leaves, and in the entire sub- 

 stance of many plants of low organization, we find tissue 

 composed of short and comparatively thin-walled cells, to 

 which the name parenchi/ma has been given. On tho 

 other hand, in the substance of wood, in the inner bark, 

 in the petioles and veins of leaves, etc., we meet with 

 tissue consisting of long, pointed and overlapping cells, 

 and known as 2)ro.se,nt/ii/)ii(i. That of tho wood \s fibrous 

 tissue, and that of the inner bark is tlie bast, specially 

 characterized by the extraordinary length and flexibility 

 of the cells. Sclercnchyma and collcuchyina have 

 . already been referred to. In the former the cells are 

 commonly, though not always, short ; while in the latter 

 they are usually long, but the ends are not pointed. 



