412 



BOTANY 



PART II 



cells ; this is a by-product of the process of assimilation. Among the 

 more highly developed forms the thallus exhibits a well-differentiated 

 anatomical structure. The outer cell layers, as a rule, function as 

 an assimilatory tissue, the inner cells as storage reservoirs. In the 

 Laminariaceae and Fucaceae structures closely resembling the sieve- 

 tubes of the cormophytes occur, and conduct albuminous substances. 



Even in the largest Sea-weeds (including the Red Algae) intercellular spaces 

 containing air are wanting in the tissues. According to KNIEP the gaseous inter- 

 changes in assimilation, and especially in respiration, are on this account difficult 



in the more massive Algae. On the other hand, 

 gases readily diffuse through algal cell walls. 



Four orders of the Phaeophyceae may 

 be distinguished. The Phaeosporeae include 

 forms with simple structure of the thallus 

 which is frequently filamentous. They are 

 vegetatively reproduced by means of zoo- 

 spores and sexually by ciliated gametes. 

 They thus resemble the simpler Green Algae. 

 The Tilopteridaceae and Dictyotaceae stand at 

 a higher level, their sexual cells being differ- 

 entiated as large non-motile egg-cells and 

 small ciliated spermatozoids. Their sexual 

 cells are formed on special sexual individuals 

 or gametophytes. From the fertilised egg 

 the asexual generation or sporophyte is 

 FIG. 353. -.4, pieurodadia locust, developed ; this is similar to the gametophy te, 

 Uniiocuiar sporangium with its but produces the asexual spores so that there 

 SStrrS.^ is a "^-marked alternation of generations 

 chromatophore. (After RLE- which is also apparent in some of the Phaeo- 

 BAHN.) B, Chorda fiium. Zoo- sporeae. In the Laminariaceae also there is 



spores. (After REINKE.) (From i i, - 



OLTMANNS' Algae.) oogamy and a regular alternation ot genera- 



tions, but the sporophyte and gametophyte 



are very unlike, the latter being a small filamentous dwarf plant. 

 The Fucaceae are also characteristically oogamous, but produce no spores 

 and thus lack an alternation of generations. 



The zoospores, gametes, and spermatozoids are spindle-shaped and always have 

 a red eye-spot and two laterally inserted cilia, one directed forwards and the other 

 backwards. They have a great resemblance to certain brownish-yellow Flagellata. 



Order 1. Phaeosporeae 



In this order are included the majority of the Phaeophyceae. A sexual multi- 

 plication is effected by means of swarm -spores, which are produced in large numbers 

 in simple (unilocular) sporangia and germinate shortly after swarming (Fig. 353). 



In addition to unilocular sporangia, multilocular sporangia are produced in the 

 Phaeosporeae (Fig. 354). Each cell of the multilocular sporangium produces a single 

 swarm-spore, rarely several. The conjugation of these swarm-spores has been 

 observed in some genera. On this account these swarm-spores must be termed 



