560 LIFE-HISTORIES 



or certain parts of them, to further protect the eml)ryo and pro- 

 vide for its surer and wider dispersal. Hence arise protective shells 

 (Figs. 23-36) and various aids to dissemination, including wing-like 

 or plume-like appendages (Figs. 55, 59, 76, 159, 197, 215, 248, etc.), 

 for catching the wind; elastic springs for propelling the seeds to a 

 distance (Figs. 164, 165), and succulent parts (Figs. 88-110) which 

 being attractive food, lead animals to swallow indigestible seeds 

 and transport them often to enormous distances. Thus, it is in 

 plants which form seeds within a case that we find the most perfect 

 provision for the welfare of offspring; and it is doubtless because 

 of this provision that angiosperms are the dominating plants of 

 to-day. 



In concluding our survey of vegetable evolution it may help us 

 to a just perspective if we briefly review the main steps which the 

 ancestors of angiosperms appear to have taken in their long upward 

 journey. The accompanying diagrams (Fig. 384) will serve to 

 recall the chief facts and conclusions already presented regarding 

 the phjdogeny of these highest plants, and also the ontogeny of 

 modern types representing the supposed links in the series. In 

 accordance with the "law of recapitulation "(see page 435) we find 

 that the younger the stages the more they are alike, and that imma- 

 ture stages of the higher types correspond to mature stages of lower 

 types, the youngest stage of each being a single cell like the supposed 

 ancestor of all. This reproduces simply by fission, a process which 

 is retained by all higher forms as growth by cell-division resulting 

 in cell-rows, cell-plates, or cell-masses variously differentiated into 

 tissues and ultimately, tissue-systems. Vegetative reproduction 

 occurs also in these higher forms through the occasional separation 

 of cells or cell-groups cai)able of independent life. Sexual reproduc- 

 tion appears with the fusion of two protoplasts to form one which 

 afterward increases by division. In the aquatic forms the fusing 

 protoplasts soon became motile and this motility is long retained 

 in the male by their descendants while adapting themselves to a 

 terrestrial life, and disappears finally when this is fully attained. 

 The single protoplast resulting from tlie fusion of two gives a second 

 unicellular beginning and thus the life-history of an individual 

 becomes divided into a sexual and a non-sexual stage or generation. 

 When the female ]:»rotoplast remains attached to a plant in the 

 sexual stage until after fertilization there results an egg-cell wliich 

 becomes a non-sexual embryo if the connection be maintained so 

 that the sexual generation may nurse the non-sexual. This new 

 begiiming, nursed by the more primitive stage, affords, it would 

 seem, a good opportunity for the transition from life in water to life 

 on land. Then, too, the nursing, when not excessive, both permits 

 and encourages the highest development of the non-sexual genera- 

 tion. Finally, it nurses the nurse, and thus through ample provision 

 for both nurse and nursling produces a seed well cared for in every 

 way. This is the greatest achievement of the vegetable kingdom. 



