MICROSCOPIC FORMS OF VEGETABLE LIFE. 



227 



is frequently destitute of color; and the partial solidification of its surface 

 gives it the character of an ' ectoplasm.' The nucleus, as already men- 

 tioned, is frequently absent from the cells of the lower Protophytes. It 

 is an extremely curious feature in the cell-life of certain Protophytes, that 

 they not only move like Animalcules by cilia or flagella, but that they 

 exhibit the rhythmically-contracting vacuoles which are specially charac- 

 teristic of Protozoic organisms. 



226. So far as we yet know, every Vegetable Cell derives its existence 

 from a pre-existing cell; and this derivation may take place (in the ordi- 

 nary process of growth and extension, as distinguished from ' sexual 

 generation') in one of two modes: either (1) binary subdivision of the 

 parent-cell, or (2) free cell-formation within the parent-cell. The first 

 stage of the former process consists in the elongation and transverse 

 constriction of the nucleus; and this constriction becomes deeper and 

 deeper, until the nucleus divides itself into two halves (Fig. 139, B, a, a'} . 

 These then separating from each 

 other, the endoplasm of the pa- 

 rent-cell collects round the two 

 new centres, so as to divide itself 

 into two distinct masses (c, a, a')', 

 and by the investment of these 

 two secondary 'endoplasms,' first 

 with ' ectoplasms, 'and afterwards 

 with cellulose-walls, a complete 

 pair of new cells (D, a, a') is 

 formed within the cavity of the 

 parent-cell. The latter process, 

 which is very common among 

 Protophytes (being that by which 

 'zoospores,' or ( swarm-spores,' 

 are commonly produced, 245), 

 is chiefly seen among Phanero- 

 gams in the production of a num- 

 ber of cells at once within the 

 cavity of the ' embryo-sac ' ( 349), 

 which may itself be considered 

 as a distended parent-cell. The 



endoplasm, in the former of these Duplicative Subdivision of Cells in Endosperm 



Cases, instead Of dividing itself of Seed of Scarlet-runner:-A .ordinary cell, with 



n n & T nucleus a, and nucleolus 6, imbedded in its proto- 



llltO tWO halves, Usually breaks Up plasm;-B, cell showing subdivision of nucleus into 

 in+r nnmprrma fipcrmpnfa nnrvp two halves, a and a'; c, cell in saro.e stage, showing 

 O numeipUS Segments COrie- contraction of endoplasm (produced by addition of 

 Spending With One another in Size water), into two separate masses round the two seg- 

 and form '"fiMrr 1/LQ\ oanV. ^ ments of original nuclBUS^D, two complete cells 



/TTirr 1/1Q\ ^oo^ ^ , 



(.tig. 149), each Ot within mother-cell, divided by partition. 



which escaping from the parent 



cavity becomes an independent cell, and gives origin by duplica- 

 tive subdivision to a new fabrio. In the second case, the endoplasm 

 groups itself, more or less completely, round' several centres, each of 

 which may or may not contain a nucleus in the first instance; and these 

 secondary cells, in various stages of development, lie free within the 

 cavity of the parent-cell, imbedded in its residual endoplasm, each 

 proceeding to complete itself as a cell by the formation of a limiting wall, 

 and by the development of a nucleus if hone was previously present 

 (Fig. 140). Now, in this second case, as the new brood of cells continues 

 to form part of the fabric in which it originated, its production is clearly 



