PLANT-FORMS AS COMPLETED STRUCTURES. 



1.— PROGRESSIVE STAGES IN COMPLEXITY OF 

 STRUCTURE FROM UNICELLULAR PLANTS TO PLANT-BODIES. 



Though all plants are mortal, they have the capacity of renewing themselves 

 and rejuvenating, so that, in spite of their perishable nature and limited 

 duration, the species now existing on the earth are in no danger of extinction. 

 The rejuvenescence is always effected by means of the protoplasm of a single 

 cell ; i.e. by a small mass of slimy substance which can only be perceived by 

 the naked eye in the rarest instances on account of its minute dimensions. 

 The largest palm in its rejuvenescence must pass through this unicellular stage 

 exactly in the same way as the smallest of mould-fungi. The only difference is 

 that in large and usually long-lived plants a longer time elapses before this 

 stage is reached, while in small plant-forms many generations may pass away 

 and be replaced in the course of a single year. The protoplasm in the minute 

 rejuvenating cell always grows at the expense of its surroundings, moulds itself 

 in the manner peculiar to its species, and divides when it has attained to a 

 certain size into two or more masses, which have inherited the capacity of 

 dividing afresh. 



Each one of these protoplasmic masses is to be regarded as an individual. 

 When the adjacent masses of protoplasm, the result of continued division, remain 

 in connection with one another, as indeed seems to be usually the case, each 

 retains a certain degree of independence; nor, should a severance take place, 

 are they necessarily abandoned to destruction. Under favourable conditions 

 they may, although separated from their companions, enlarge, divide, and 

 continue to grow. In not a few unicellular plants it is even customary for 

 each mass of protoplasm immediately after its formation to separate itself 

 entirely, and, for the future, to live independently. It is remarkable that 

 in most of these unicellular plants a time arrives, i.e. the time of pairing, 

 when they again seek each other with the view of uniting; but this period is 

 of short duration compared with the length of the isolated life. Moreover, 

 a definite bond of union has been recognized between the separate individuals 

 produced from one cell. Just as caterpillars which creep out of the eggs 

 laid by a butterfly are seen not to disperse, but to follow common paths and 

 ways, so the swarm -spores of Sphcerella pluvialis swim together in groups 

 from one place to another, and select a suitable spot for settling down. The 



