526 SUMMARY OF CUERENT RESEARCHES RELATING TO 



are in tlie same way the products of tlie starch-generators occurring 

 in the growing point of the radicle. A very good example of spherical 

 leucoplastids is afforded by the transparent cells of the growing point 

 of Impatiens parvijlora ; also by those of Trop(eolum majus and Dahlia 

 variaUUs. The plastids in the growing points of monocotyledons 

 appear as a rule to be very small ; they can readily be made out in the 

 aerial roots of Har'icegia comosa and in Tradescantia. 



An exceedingly interesting point is that the plastids of seedlings 

 are not formed during germination, but are already present in the seed, 

 and that those found in the growing points of the radicle and plumule 

 are the direct product of similar bodies contained in the ovum-cell, 

 and therefore are directly derived from the parent plant. The cell- 

 contents of the embryo are, however, far too dense for the plastids to 

 be detected in them. 



One and the same plastid may undergo many metamorphoses ; the 

 leucoplastids may become chlorophyll-grains, and subsequently again 

 lose their colouring matter ; chloroplastids and leucoplastids may 

 become chromoplastids. It is clear that the chlorojilastid must be 

 regarded as the original form of plastid, from which the others have 

 subsequently developed. The lowest organized plants known to con- 

 tain leucoplastids or chromoplastids are the Characese, the former in 

 their apical cells, the latter in their antheridia. 



Leucoplastids owe their origin either to previously existing leuco- 

 plastids or to chloroplastids ; this latter is the case with those flowers 

 and fruits which change colour from green to white, as the berries of 

 the " snowberry tree." They do not appear here to have any special 

 function. In other cases they are the generators of starch. On the 

 other hand, in many flowers leucoplastids become transformed into 

 chromoplastids. 



Chloroplastids (those which contain chlorophyll) are also derived 

 either from other chloroplastids or from leucoplastids. To this class 

 the author refers with hesitation the red and brown assimilating struc- 

 tures of the Florideae and Phteosporefe, as well as the brown pigment- 

 bodies of Neottia nidus-avis. 



Chromoplastids exhibit every variety of shade between pure 

 carmine-red and greenish yellow ; other colours, as blue, do not 

 appear to occur, though vacuoles and other bodies have been mistaken 

 for them. They are sometimes more or less regularly spherical, 

 more often of a crystalline form, but most often fusiform, acicular, or 

 red-shaped. As long as they are uninjured they never contain 

 vacuoles. They are invariably derived from leucoplastids or chloro- 

 plastids, even the form of these being sometimes not changed. Some- 

 times they assume regular crystalline forms. They may be classified, 

 according to their forms, under three types:— (1) nearly or quite 

 spherical, (2) with two or more pointed ends, and (3) rod-shaped with 

 rounded ends. The first type occurs in the aril of the yew, the berries 

 of Solanum didcamara, and the petals of Nupliar luteum. Those of the 

 second type vary greatly in form, and are found in the perianth of 

 Hemerocallis fulva and Asphodelus Juieus, the fruit of Sorhus aumparia 

 and Euonymus enropceus, the petals of Troj^ceohim majns, &c. We find 



